Infectious process
Learning objectives:
After completing the course, the students will be able to:
1. identify the six requisites for the perpetuation of a communicable disease (the cycle of
infection)
2. explain mechanisms of disease production
3. define reservoir of infection and classify human reservoirs
4. define carrier and list its types
5. identify the portals of exit of infection from the reservoir, and give an example for each
6.
identify different modes of transmission of communicable diseases, and give an example
for each
7. recognize the various defense mechanisms of the human body, and give an example for
each
8. define herd immunity, and list the factors that play a role in community protection
9. define primary, secondary and tertiary levels of prevention
10. outline the general principles of prevention and control of communicable diseases
Learning Activities
Solving exercises and M.C.Q’s on cycle of infection
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Definitions:
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Infections: It is the entry, development and multiplication of an infectious agent in the
body of man or animal. Infection is not synonymous with infectious disease; the result of
infection may be inapparent or manifest infectious disease.
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Infectious disease: A clinically manifest disease of man or animal resulting from
infection.
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Non- infectious disease: A disease is not due to infectious agent as diabetes and
cardiovascular diseases.
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Communicable diseases: It is an illness caused by an infectious agent or its toxic
product which can be transmitted directly or indirectly or through vector from the
reservoir to a susceptible host.
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Non- communicable disease: It is an infectious disease which cannot be transmitted
from the reservoir to a susceptible host.
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Contagious disease: A disease that is transmitted through contact. E.g. scabies,
trachoma and leprosy.
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Contamination: The presence of living infectious agents on the exterior surface of the
body or on the clothes or articles of the person or on any inanimate object in the
environment including water and food.
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Epidemic: It the occurrence in a community or a region of a group of illness of similar
nature, clearly in excess of its normal expectancy.
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Endemic: A disease that has established itself permanently in a certain locality or
community all the time, e.g. bilharziasis in Egypt.
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Pandemic: The appearance of a disease in an epidemic form affecting countries
sequentially (at the same time).
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Outbreak: A more or less localized epidemic affecting certain large number of a group, in
the community, e.g. outbreak of food poisoning in an institution.
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Sporadic: means scattered about. The cases occur irregularly, haphazardly from time to
time and generally infrequently. Cases are few and separated widely in space and time
showing no connection to each other.
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Opportunistic infection: This is infection by an organism that takes the opportunity
provided by a defect in the host defense mechanism e.g. AIDs, Toxoplasmosis.
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Iatrogenic disease: It is a physician induced disease.
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Eradication: Termination of all transmission of infection by extermination of the infectious
agent. ( Termination of infection from the whole world) e.g. Smallpox
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Period of communicability:
The time during which the infectious agent could be
transmitted directly or indirectly from the reservoir to a susceptible host.
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Nosocomial infection: (Hospital acquired infection). They are infections that
acquired by the patients during or associated with delivery of health care which
are not present or incubating at admission.
The requisites for the perpetuation of communicable diseases:
There are six requisites for the perpetuation of the communicable disease:
(The cycle of infection)
1- Presence of microbiological agent.
2- Presence of reservoir.
3- Portal of exit.
4- Mode of transmission.
5- Portal of entry ( inlet )
6- Presence of susceptible host
I. Microbiological agent:
Microorganisms responsible for the causation of communicable diseases are classified into:
a) Viruses: These are the causative agents for a large number of diseases such as influenza,
mumps, chiken pox, poliomyelitis…etc.
b) Bacteria: Cocci ( streptococci, staphylococci, diplococci ), Bacilli ( diphtheria, salmonella,
shigella ) and vibrio (classical vibrio, Eltor vibrio)
c) Rickettsia: The causative agents of a group of infectious diseases including the typhus
group of fevers.
d) Fungi: Including Candida.
e) Protozoa: One cell animal form such as entameba causing dysentery, plasmodia causing
malaria,.
f)
Parasites: Such as schistosoma, ancylostoma ….etc.
Mechanisms of disease production ( pathogenesis ):
a) Invasiveness.
b) Toxicity.
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c) Hypersensitivity.
a) Invasiveness:
Is the ability of the organisms to invade the tissues and multiply. Each pathogen has the
ability of invasiveness and toxigenicity (e.g. treponema pallidum, typhoid and paratyphoid
organisms have a high power of invasiveness but they have low toxicity).
b) Toxicity: Toxins are:
Exotoxin: They are released by living organisms, destroyed rapidly by heating (above 600
C), highly immunogenic and converted to antigenic, non toxic toxoid by Formalin, heat and
acid. They are diffusible, don't produce fever e.g. (Neurotoxins of tetanus and botulism,
erythrogenic toxins of scarlet fever).
Endotoxin: They are toxins released after disintegration of microorganisms, highly stable i.e.
withstand heating above 600C, weakly immunogenic, not converted to toxoid, usually produce
path physiologic effects as fever, leukopenea, hypotension hypoglycemia and shock.
c) Hypersensitivity :
It is an allergic state of the host following exposure to certain antigens of microorganisms
(e.g. mycobacterium tuberculosis), whereby subsequent exposure results in a disease state.
Factors affecting disease production in relation to agents:
1. Pathogenicity and virulence:
Pathogenicity: is the ability of the organisms to produce specific clinical reaction after infection
has occurred, however, it does not refer to severity of reaction. Virulence: Is the ability of
organisms to produce severe pathological reaction, it refers to severity of the reaction.
It can be measured by:
a) Ratio of clinical to sub-clinical cases =
Clinical cases
Subclinical cases
b) Case fatality rate =
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Number of deaths due to a particular disease in an area and year
Total number of cases of this particular disease in the same area and year
 100
2. Antigenic power of the microorganisms:
The ability to initiate the development of antibodies or antitoxin and associated immunity. It can
be measured by:a) Second attack frequency: In certain diseases second attacks are rarely recorded as in case
of measles, mumps and Chicken pox. In other diseases re-infection occurs as in case of
common cold, upper respiratory infections, syphilis and gonorrhea.
b) Age specific attack rate: In diseases caused by microorganisms of high antigenic power as
measles there is a drop of the attack rate after young age.
3. Period and ease of communicability:
It can be measured by the secondary attack rate, which is the number of secondary cases,
occurring within the accepted incubation period following exposure to a primary case,
calculated as a percentage of the number of exposed susceptible.
4. Dose of infection (inoculums): The higher the dose of infection, the more liability of having an
apparent illness, and the severer will be the diseases.
5. Tissue selectivity (tropism): Is the inherent capacity of the pathogen to invade some particular
tissues. This is the factor that gives each disease its characteristic symptoms and signs.
6. Host specificity: Some pathogens infect man only as relapsing fever, other infect animals
only, while some others infect both man and animal such as zoonotic diseases.
7. Spore formation: The ability of some bacteria to change to a resistant form under unsuitable
conditions and these spores remain viable for long periods.
When these spores get the chance of coming into contact with a susceptible host-under
favorable condition, they change to vegetative forms, they cause the disease as in case of
tetanus and anthrax.
8. Viability of the organism (the resistance of the organism): Is the ability to live outside the
body, the longer the duration, the more the chance to come into contact with a new host
transmitting the infection to them.
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9. Susceptibility of the pathogen to chemotherapy: The degree of sensitivity to antibiotics differs
from one pathogen to the other and even from one strain of the same pathogen to another.
II. Reservoir of infection:
Definition:
This is the place or depot where the infective agent survives, grows and multiplies in
such a manner that it can be transmitted to a susceptible host. Reservoirs may be man, animal,
plant or soil (or combination of these)
Man is the most common reservoir of infections followed by animals.
Human reservoirs:
Human reservoirs of infection may be in the form of cases or carriers.
a) Cases:
Whether these are typical cases or missed (inapparent or subclinical)
Inapparent infection: The presence of infection in a host without recognizable clinical signs
or symptoms. They are identifiable only by laboratory means or positive skin tests.
Severe cases are not necessarily more communicable than mild subclinical cases. From the
public health point of view, mild and subclinical cases may be more important for the spread
of infection being unrecognized and overlooked.
b) Carriers:
A carrier is defined as a person or animal that harbors the infectious organisms and is
apparently free from clinical manifestations of the disease, and serves as a potential source of
infection.
Carriers are dangerous because:
1- They do not show any clinical manifestation.
2- The carrier and his contacts are not aware of their condition.
3- It is difficult to discover them.
4- It is not always possible to deal with them.
5- The long duration of carriage in some communicable diseases (Typhoid, HIV).
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6- The high prevalence of carrier state among the population.
Carriers are classified according to :
1. Place of carriage:
a) Upper respiratory carriers as in case of diphtheria, streptococcal and meningococcal
infections.
b) Fecal carriers: In case of typhoid, paratyphoid, cholera, infectious hepatitis.
c) Urinary carriers: In case of typhoid and paratyphoid.
d) Skin carriers: As in case of staphylococcal infection.
2. Duration of carriage:
a) Transient carrier: A person who harbors and excretes the infectious organisms up to weeks.
b) Temporary carrier: A person who harbors and excretes infectious organisms up to 3 months
(one year for enterica)
c) Chronic carrier: A person who harbors and excretes organisms for more than 3 months (more
than one year for enterica)
d) Permanent carrier: A person who harbors and excretes the infecting organisms for life.
Most of the carriers (about 95%) are of the temporary type.
3. Chronologically (according to the spectrum of infection) carriers are classified into:
a) Incubatory carrier: A person who harbors and excretes the infecting organisms during the
incubation period. For example; hepatitis A, mumps and poliomyelitis.
b) Contact carrier: Contact with an infected person as doctors, nurses, parents as well as
servants. Contact carriers are of transient type, usually the period of carriage ends as soon
as the patient is cured or the contact is over. Contact carriers are common in cholera and
typhoid.
c) Convalescent carriers: A person who discharges the microorganisms during convalescent
period for example typhoid. Here there will be a need for carrying out 3 consecutive
bacteriological examinations before release of the cases.
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Animal reservoirs:
Animals can act as reservoirs, whether in the form of diseased or carriers. Zoonosis:
Infectious disease transmissible under natural conditions from vertebrate animals to man.
The important animal reservoirs are cattles in bovine T.B, goats in Brucellosis, dogs in
rabies and rats in plague. Mice, rodents, ducks and cows in salmonella, and monkeys in
yellow fever.
There are different modes of transmission of diseases from animals to man:
1. Direct contact with animal products and excreta: Brucellosis, anthrax, Toxoplasmosis,
rabies
2. Common vehicle; Consuming animal products as (bovin TB, salmonella , Brucellosis
,anthrax , Toxoplasmosis)
3. Vector: Aedes Aegypti mosquitoes (Yellow fever), Anopheles mosquitoes ( malaria) and
Flea ( Bubonic plaque)
4. Direct droplet: Inhalation anthrax
Inanimate reservoir:
Soil as in case of tetanus .
The source of infection :
This term refers to the immediate source of infection, i.e. secretion or excretion, in which the
organism is ready to be transmitted to a susceptible host. The source of infection may not be a
portion of the reservoir. For example, man is the reservoir of shigella infection; a cook who is a
carrier may infect food which is considered as the source of infection. While soil acts as both the
reservoir and source of infection in tetanus.
III. Portal of exit:
The modes of exit from reservoirs are one of the followings:
i) Respiratory tract:
Organisms will leave the body via the mouth and nose in coughing, sneezing, laughing or
even talking. This is the portal of exit in measles, whooping cough, diphtheria, streptococcal
sore throat, influenza, common cold, mumps…etc.
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ii) Gastrointestinal tract :
The infecting organisms are liberated either in feces as typhoid, paratyphoid, cholera, eggs
of ascaris, …etc. Or through the vomitus as in case of cholera. In certain diseases where the
primary sites of infection is the bowel, the organisms will pass through the faeces e.g.
poliomyelitis ( affects CNS ) and infectious hepatitis .
iii) Urinary tract :
This occurs in some diseases where infection is general and organisms are found in blood
as in case of typhoid, undulant fever, bilharziasis as well as, in local infection of genito-urinary
tract as in case of gonorrhea.
iv) Skin and mucous membrane:
as in case of erysipelas and impetigo, also, discharges from mucous membrane as in
purulent conjunctivitis, and venereal diseases.
v) Insect bite:
As in case of typhus, plague and malaria.
vi) Syringes and taking blood from donors:
As in case of viral hepatitis.
vii) Utero-transmission ( Trans placental ):
Across the placenta from maternal blood to foetal circulation such as AIDS, Syphilis and
German measles.
IV. Mode of transmission:
Studying the modes of transmission in any communicable disease is of great importance for
prevention and control of diseases.
The modes of transmission could be classified into:
A) Contact transmission:
i) Direct contact.
ii) Indirect contact.
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iii) Droplet contact.
iv) Transplacental.
B) Common vehicle transmission, which can occur through :
i) Ingestion.
ii) Inoculation.
Iii) Deposition.
C) Vector transmission:
i) Mechanical.
ii) Biological.
D) Air-born transmission:
i) Droplet nuclei.
ii) Dust particles.
A) Contact transmission:
Contact transmission means transmission during an association between the infected man
or animal and the new host. This association (contact ) may be in the form of :
1. Direct contact:
In this situation, there is an association between the infected man or animal and the new
host, without a third object. Direct contact may occur through sexual intercourse, kissing and
touch. Examples for direct contact transmission are venereal diseases, scabies, contact of
saliva of a rabid animal with abraded skin.
2. Indirect contact:
In this mode of transmission, the spread of infection is through touching contaminated
objects for instances, toys, handkerchiefs, soiled clothing towels. Microorganisms will be
transmitted from hands to mouth or from hands to abraded skin or mucous membranes,
examples are conjunctivitis, trachoma, skin infections, and diphtheria. Also, this mode of spread
may be of importance in spreading hospital infections particularly in transmitting infections to
surgical wounds.
3. Droplet transmission:
Droplets spray onto the mucous membrane of the nose, mouth during sneezing coughing,
spitting or talking ( usually limited to a distance of one meter or less). These droplets will pass
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into the nose or mouth of the host directly. Examples: Measles, streptococcal infections,
influenza, diphtheria
4. Trans placental transmission:
In this type of contact transmission the organism is carried from mothers to fetus through the
placenta, e.g. Syphilis, AIDS and German measles.
B) Common vehicle transmission
Vehicle of transmission can be water, milk or biological products as blood, agents via this
vehicle are introduced to the susceptible host through:
1. Contaminated foods or drinks: ingestion of contaminated food and drink
for instance typhoid, paratyphoid, food poisoning, dysentery and cholera.
2. Infected blood and plasma: This is important in case of inoculation of plasma, blood, serum or
even vaccine, e.g. viral hepatitis and syphilis.
3. Agents of diseases inoculated to the skin or mucous membrane :
This is important for the group of diseases that can be transmitted by the swimming pools,
e.g. Conjunctivitis and sinusitis .
*N.B.: The role played by common vehicles depends on the viability of infective agents outside
the body, environmental influences as dryness, temperature and sunlight.
C) Vector transmission:
Various insects are known to be vectors for disease transmission. There role is either
mechanical or biological.
1- Mechanical transmission: The vectors carry pathogenic organisms of different infections on
the feet or mouth parts, or may be ingested and pass in the insect feces or vomited later on.
The mechanical transmission can be:
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a) Direct:
If the insect as houseflies become contaminated from discharges of infected eyes and transmit
it to a healthy eyes as in case of purulent conjunctivitis.
b ) Indirect:
This occurs when insects as houseflies and cockroaches carrying pathogenic organisms settle
on human food or drink, they will contaminate such food as occurs with typhoid, dysentery and
cholera.
1- Biological transmission:
The agents have to pass through some biological activity inside the vector which requires a
certain period of time " extrinsic incubation period ". After this period, the insect is then able to
transmit infection to the new host. The infectious agent may be passed vertically to
succeeding generations (trans-ovarian transmission ).
The biological vector transmission can be:
1- Propagative biological transmission: Simple multiplication of the causative
organism in the vector. The best example is yersenia pestis agent of plague in flea
and virus of yellow fever in the Aides Aegypti mosquito.
2-Cyclo-propagative: Organisms here multiply and undergo changes within the vector as in
case of malaria.
3-Cyclo-developmental : Here the agent of disease have a cycle of developmental changes
inside the vector, with no multiplication, example is filaria in mosquito.
D) Air-born transmission:
The dissemination of suspended particles in the air consisting partially or wholly of
microorganisms. They remain suspended for a long period of time (the reservoir and the
host may not be in the same room). Some organisms retain their infectivity (as TB and
hemolytic streptococci) and the others do not. Air-born transmission can be :-
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1. Droplet nuclei: The small particles result from evaporation of the fluids from the droplets
of infected person. They contain pathogens and remain suspended in air for a long
period of time. When they are inhaled by a susceptible host they cause infection.
2. Dust nuclei:- The large particles fall on the ground to mix with the dust and become part
of the dust of the room. The organisms resist drying for a long time ( TB and hemolytic
streptococci ). The source of these particles is the discharge of infected persons
contaminating bedding, clothes, floor, soil, …
V. Portal of entry (Inlet)
With some exception, the portal of entry to a host corresponds the portal of exit from reservoir.
Accordingly, we can classify the portals of entry into:
a. Natural inlets including mouth, nose, rectum, vagina, urethra and conjunctiva.
b. Unnatural inlets for instance by inoculation through blood sucking insects or by injection.
Incubation period:
The time interval between exposure to an infectious agents and appearance of the first sign or
symptom of the disease. There is an average period for every communicable disease. However,
there is a range. The length of incubation period depends on:
1- If the agent is an organism or toxin.
2- The virulence of the organism.
3- The dose of the inoculums.
4- The host resistance.
Long incubation periods are encountered in infectious and serum hepatitis, tuberculosis and
AIDS. Short incubation period is that of staphylocooal food poisoning, as the agent is the
enterotoxin present in the ingested food rather than the organism.
Knowledge of the incubation period is important to :1- It is used practically for surveillance of contacts of communicable diseases.
2- To apply preventive measures in certain diseases.
- Measles vaccine if given to the contacts in the first three days after exposure it will
prevent the disease.
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- If given in the second three days after exposure it will modify the attack ( subclinical
manifestation and result in solid immunity )
3- Identification of the source of infection, water borne epidemic of
typhoid has longer
incubation period than milk borne while shell fish borne typhoid has a shorter one.
Extrinsic incubation period:This is the period taken by the infective agent outside the human body until it becomes infective
again to a new individual, e.g. yellow fever takes (9 – 12) days in Aedes Aegypti to be infective
after the blood meal .
VI. The susceptible host
The host is a person or other living animal, including birds and arthropods, that affords
subsistence or lodgment to an infectious agent under natural condition.
Resistance:
It is the total body mechanisms which act as barriers to invasion or multiplication of infectious
agents or their damaging effects of their toxins.
Types of resistance:
A) Natural barriers ( inherent resistance, non specific ).
B) Acquired resistance ( immunity, specific ).
A) Natural barriers:
It is non-specific resistance of the body against the invading organisms which does not
depend on the presence of specific antibodies or antitoxin for protection, but depends on the
anatomical or physiological characteristics of the host.
b) Acquired resistance:
This depends on antibodies production. Immunity may be :14
1- Passive immunity.
a) Natural.
b) Artificial.
2- Active immunity.
a) Natural.
1. Passive immunity:
b) Artificial.
Passive immunity is the type of resistance in which ready made
antibodies are gained.
a) Natural passive immunity ( infant immunity ): It is the resistance of infant due to the
presence of antibodies passed from the mother to the foetus via the placenta. The mother
should have acquired the infection and / or vaccine and developed specific antibodies
against certain diseases. These passive acquired antibodies in the foetus are at their
highest level at birth, then start to decline gradually until they are insignificant usually by the
end of six months. Such natural passive immunity in the infant could be induced artificially
by immunizing the mother during pregnancy, as in case of immunization by tetanus toxoid to
protect the newly born infant against tetanus neonatorum. Breast milk, specially the
colostrums, contains plenty of antibodies (immunoglobulin account for about 95% of the
protein in colostrums). Antibodies are continually secreted in breast milk but in a lower level
than that of colostrums.
b) Artificial passive immunity (passive immunization): It is the immunity induced by injecting
immune serum or immunoglobulin. It is of short duration, remaining for about three weeks,
during which the antibodies given are gradually eliminated. Examples of materials used for
passive artificial immunity:
1- Sera of artificially immunized animals, these are used either for prophylaxis or treatment
as in case of antitetanic and antidiphtheretic sera.
2- Immunoglobulin: This is a plasma protein fraction that carries most of antibodies present
in the body, used as prophylaxis, e.g. in measles and infectious hepatitis.
2. Active immunity: This is the type of immunity in which the person makes or develops his
own antibodies. It may be :
a) Natural actively acquired immunity or post infection immunity: It may be solid or for a long
time as in case of mumps and measles or it may be of moderate duration (for years) as in
case of meningitis or it may be for a short period as in common cold.
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b) Artificial actively acquired immunity: This is produced artificially by active immunization
using an immunizing agent which is a specific antigen when introduced in the body, provoke
the formation of antibodies.
The ideal immunizing agent should be:
1- Antigenically stable.
2- Give durable immunity.
3- Have minimal side effects.
4- Easy in administration.
5- Of reasonable cost as well as being available.
6- Keeping quality is good.
Types of immunizing agents ( vaccines ):
I- Live attenuated vaccines:
a) Attenuated vaccine: In these vaccines, microorganisms lose their pathogenicity but retain
their power of multiplication and antigenicity. Attenuation can be done by repeated subcultures
or by cultivation under unfavorable condition, e.g. Sabin vaccine of polio, measles, German
measles and mumps vaccines.
b) Variant forms of living organisms vaccine: In these vaccines a milder species of the
organisms closely related Antigenically to the human disease agents, are used, such as
smallpox vaccine using cow pox virus and BCG vaccine using bovine tubercle bacilli.
II- Non living vaccines:(A) Killed or inactivated:
- Killed bacterial organisms ( using heat or chemicals as ether and Formalin ) examples
typhoid, whooping cough vaccines.
- Inactivated virus as salk vaccine of polio virus.
(B) Products of organisms ( Toxoid ):
- Toxoid is the toxins after loosing their toxicity but retaining their antigenicity e.g.
Diphtheria, and tetanus toxoid.
(C) Part of organisms:
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- The subunit of hepatitis B surface antigen ( HBs Ag ) prepared from plasma of HBs Ag
positive carriers or by genetic engineering.
- Part of polysaccharide capsule of Nisseria meningitides used as a vaccine against
meningococcal meningitis.
Herd immunity: It is the state of immunity within the community. Herd immunity is the factor
that decides the epidemiological pattern of any infectious disease among that community. The
incidence of diseases rises at times when the number of susceptible in the population is highest
and the herd immunity is lowest. The best example is measles epidemic in Fidjii in 1975 in
which the attack rate approached 100% and fatality rate was excessively high. The spread of
the disease among all age groups equally is characteristic of the absence of immunity. Also, the
well known rhythm of measles epidemic in urban communities is due to the variation in
community susceptibility. Following an epidemic, immunity is at its highest level. The level of
susceptibility increases as new infants are born, an epidemic will develop after accumulation of
susceptible. The herd immunity could be produced artificially by immunization, or naturally after
infection.
Several factors play a role in community protection namely:
1- The extent of coverage of the immunization program.
2- The degree of resistance to infection afforded by the vaccine.
3- Duration and degree of infectivity of the organism.
4- Past experience with different infections.
5- Overcrowding and environmental sanitation.
General Principles of Prevention
Successful prevention depends upon knowledge of causation and dynamics of disease
transmission, identification of risk factors and risk groups and availability of prophylaxis or early
detection and treatment.
Levels of prevention:
It has become customary to define prevention in terms of three levels:
1. Primary prevention.
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2. Secondary prevention.
3. Tertiary prevention.
1- Primary prevention.
Actions taken prior to the onset of the disease which control the causation and the risk
factors thus limit the incidence or prevent the possibility of occurrence of a disease. Example of
primary prevention:
1- Health education.
2- Environmental sanitation.
3- Nutritional interventions.
4- Specific protection.
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Immunization.
-
Chemoprophylaxis.
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Use of specific nutrients.
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Protection from carcinogens and allergens.
-
Protection against accidents
-
Protection against occupational hazards.
2- Secondary prevention.
Actions hinder the progress of a disease at its early stage and prevent complications. The
specific interventions are early diagnosis (e.g. Screening tests and case finding programs) and
adequate treatment before irreversible pathological changes have taken place. It may also,
protect others in the community from acquiring the infection and thus provide at once secondary
prevention for the infected individuals and primary prevention for their potential contacts.
Secondary prevention is largely the domain of clinical medicine. The health programs initiated
by governments are usually at the level of secondary prevention. Early diagnosis and treatment
though not as effective and economical as primary prevention, it may be critically important in
reducing the high morbidity and mortality in certain diseases such as essential hypertension,
cancer cervix and breast cancer.
An example of secondary prevention is mass treatment. A mass treatment
is used in the
control of certain diseases, e.g. Trachoma, malaria and bilharziasis. The rationale for a mass
treatment program is the existence of at least four to five cases of latent (incubating) infection
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for each clinical case of active disease in the community. Patients with a latent infection may
develop disease at any time. In such cases, mass treatment is a critical factor in the interruption
of disease transmission.
3- Tertiary prevention:
Actions taken when the disease process has advanced beyond its early stages i.e. intervention
in late pathogenesis phase. The aim of tertiary prevention is :
- To limit impairments and disabilities.
- To decrease suffering.
- To promote the patient's adjustment to rehabilitation.
 Rehabilitation:- It is a measure to train disable individuals to reach the highest level of
functional ability by using combined coordinated medical, social and educational
measures.
Examples of rehabilitation:1- Special schools for blind pupils.
2- Provision of aids for crippled.
3- Reconstructive surgery for leprotics.
4- Modification of life for tuberculous or cardiac patients.
General Principles of Prevention
And Control of Communicable Diseases
Prevention:Measures applied to prevent the diseases before their occurrence.
Control:Measures applied to avoid the spread of the diseases after their occurrence i.e. Reduce the
incidence and / or the prevalence of these diseases.
I - Measures applied to the agent: Sterilization and disinfections.
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II – Measures applied to the reservoir of infection:
A) Measures applied to cases:
1- Case finding.
2- Reporting.
3- Isolation.
4- Treatment.
B) Measures applied to carriers:
1- Detection.
2- Exclusion from work.
3- Treatment for the carrier state.
C) Measures applied to animal reservoir.
III - Measures applied to the contacts:
1- Enlistment.
2- Surveillance.
3- Isolation.
4- Increase resistance.
5- Health education
IV - Measures applied to the environment:
- Improving Sanitation.
V - Measures applied to the host:
1- Non-specific.
2- Specific.
I - Measures applied to the agent:
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Sterilization:- Killing all forms of living organisms. It is used for medical and surgical
instruments.
Disinfections:- Killing the infectious agents outside the body by direct exposure to chemical
or physical agents:1- Concurrent disinfection:- Is the application of disinfective measures as soon as possible
after the discharge of infectious material from the body of an infected person or after the
soiling of articles with such discharge.
2- Terminal disinfection:- Application of disinfective measures after the patients has been
removed by death or to a hospital or has been ceased to be a source of infection (by
treatment). Disinfection is necessary only for diseases spread by indirect contact.
II – Measures applied to the reservoir of infection:
A) Measures applied to cases
1- Surveillance: It is the collection , analysis, interpretation and dissemination of
information about a selected health event. This information is important to plan,
implement and evaluate a health program.
2-Case finding and early detection: It is the first step in the control of a
communicable disease.
3-Reporting: Notifying an appropriate authority of the occurrence of a communicable
disease. Diseases in man are reported to the local health authority, those in animals
to sanitary, veterinary or agriculture authority. Zoonotic diseases are reported to both
authorities. Diseases under international health regulation should be reported to
WHO. The aim of reporting is to provide information to permit appropriate control
measures and to compare the frequencies of diseases occurrence between different
countries.
4- Isolation of the patient: Isolation means the separation, for the period of communicability
of the infected person, from other healthy persons, under conditions that will prevent the
direct or indirect transmission of the infectious agent to the susceptible person. Isolation is
continued for the period in which the patient is dangerous to other.
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Since the period of communicability varies in different diseases, the period of isolation must
vary correspondingly. In many instances, it is possible to determine it by exact laboratory
methods.
Isolation has a limited value, and cannot be relied upon to prevent spread of infection in a
community for the following reasons:
a- Many diseases are highly communicable during the early stage.
b-The exact period of communicability is not known in many diseases.
c- Carriers of infective agents may go undetected.
d- Many mild cases of infection spread disease without being detected.
- Isolation in a hospital or in separate quarter is required for cholera, plague
- Isolation at homes: If the condition of home is suitable patients with typhoid, paratyphoid
and meningococcal meningitis, whooping cough, poliomyelitis and infective hepatitis.
5- Treatment: Chemotherapy: The use of chemicals to cure clinically recognizable disease or to
limit its further progress. This is not only for the sake of the patient, but also, for reducing
the period of communicability, and to limit the transmission to the contacts.
B) Measures applied to carriers:
1- Detection: Detection of carriers is important in diseases in which carriers are an
important reservoir of infection, e.g. Enteric fever. However, the value of detection for
carriers depends on:
a) The proportion of carriers in the community.
b) The occupation of the carriers and its intimacy to contacts as food handlers and those
working in closed community ( e.g. School or day care centers ).
2- Exclusion from work: Must be done if his occupation is a food handler (e.g. Typhoid
carrier) or a teacher (e.g. Diphtheria carrier).
3- Treatment for the carrier state.
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C) Measures applied to animal reservoir:
Reservoir eradication is only possible in diseases transmitted from animal to man (zoonotic
diseases), e.g. Killing the infected animals( in rabies), slaughtering ( in bovine tuberculosis),
immunization (in Brucellosis), careful husbandry and sterilization of animal products (in
anthrax).
III – Measures applied to the contacts:
Contacts are persons in close association with infected persons or an
animal or material handled by an infected person. Contacts are undergoing
special risk of contracting infection. Contacts at large include household
contacts, occupational and school contacts. The following are measures for contacts:1- Enlistment: Contacts are listed and dealt with according to the disease.
2- Surveillance of persons :(put under observation) The supervision of contacts to permit
recognition of infection or illness without restricting their movements. The period of
observation is equal to the longest incubation period of the disease under observation.
Taking the temperature is the best criterion for the development of the disease.
3-Isolation: It means separation of contacts in a specific place to prevent direct or indirect
contact between them and unexposed individuals. Period of isolation is for the longest
incubation period counted from date of last exposure. This measure is only applied in case of
contacts of pneumonic plague only.
4- Increasing the resistance of the contacts: This is done either by :
a) Active immunization as in case of measles.
b) Passive immunization as in case of diphtheria.
c) Chemoprophylaxis: is administration of chemicals including antibiotics, to prevent
the development of an infection or the progression of an infection to active
manifest disease.
Examples of Chemoprophylaxis:(1) Isoniazid for contacts of tuberculous patients.
(2) Sulphadiazine or Rifampicin for contacts of meningococcal meningitis.
(3) Erythromycin or penicillin for unimmunized contacts of a diphtheria case.
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(4) Tetracycline for contacts of cholera and pneumonic plague.
(5) Penicillin for contacts of syphilitic patients.
IV - Measures applied to the environment:
This include chlorination of water, sanitary sewage and refuse disposal, rodent control,
vector and food sanitation.
V – Measures applied to the host:
a) Non-specific measures:
1- Health education.
2- Good standard of nutrition.
3- Provision of adequate housing.
4- Provision of adequate recreation facilities.
5- Provision of suitable working condition.
6- Periodic selective examination.
7- Personal hygiene.
8- Protection against accidents.
b) Specific measures:
Use of specific passive and active immunization and Chemoprophylaxis.
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Surveillance
Surveillance is the collection, analysis, interpretation and dissemination of information about a
selected health event. This information is important to plan, implement and evaluate a health
program.
Examples of health events that can be put under surveillance:
Surveillance of polio, surveillance of accident, surveillance of malnutrition and surveillance of
adverse events following immunization.
Surveillance is a dynamic process and its components are depending on each other.
The component of surveillance:
.Data collection
.Data analysis and interpretation
.Dissemination of data.
Collection & reporting of data
Analysis &interpretation
Action to control,
Of data
eliminate or eradicate
Dissemination of data
Collection and reporting of data:
Levels of Collection and reporting of data:
Data about the disease under surveillance is collected. At the primary health centers or hospital
or clinic ( by the physician) , then reported to the district level. From different districts to the
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provincial level then to the national level) Ministry of Health). For certain prescribed diseases,
report is made by the national health authority to the WHO.
- Methods of data collection :data collection is either passive or active:
a) Passive data collection: Data about the disease is collected by the physician in the primary
health care centers routinely in a regular, frequent and timely manner.
b) Active data collection: The health officer from the districts or other higher level regularly
visits the hospitals, primary health care centers or clinics to gather information about the
disease under surveillance.
Types of data collected during surveillance of a communicable disease
1- Morbidity and mortality data: The number of cases and /or deaths, the time and place of
occurrence, personal factors and determinants of the disease.
2-Laboratory results: The role of public health lab is to confirm the cases, determine the
bacterial resistance to antibiotics, the level of immunity of the population, the vaccine potency
and microbial contamination of the environment.
3-Data about the preventive and control measures: such as the number of vaccinated, the
number of houses sprayed by insecticides, the number of contacts received chemo prophylaxis.
4-Data related to the environmental conditions: the proportion of people supplied by pure water
and coverage of services for garbage and excreta disposal.
5-Data about the vector of disease: Type, habits and susceptibility to insecticides
6-data about the reservoir: Type and distribution.
7-Data about the population: Size, distribution, and composition.(demographic data).
2. Data analysis and interpretation
The data collected from different centers, clinics and hospitals are manipulated at the higher
levels to be presented in the form of tables, graphs and maps or mathematically presented.
3. Dissemination of data :
The results obtained should be disseminated into two groups:
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a) Those who collected the data at the first level to maintain their interest.
b) Those responsible for taking actions.
The uses of surveillance:
I) Identify the disease trend so that planning of preventive and control programs can be adjusted
to meet the new situation.
2) Identify, investigate and help the control of outbreaks or epidemics.
3) Identify the population at risk for certain disease or death.
4) Identify new emerging disease (example HIV infection)
5) It is useful to formulate hypothesis: This is because the case series (one of the descriptive
epidemiological methods) depends mainly on an effective surveillance .The best example is the
association between HIV infection and occurrence of pneomocystitis carenii which was
formulated by the case series
6) Evaluation of preventive and control measures of the disease under study.
Criteria for diseases to be put under surveillance:
1. It affects large number of population. (High prevalence or incidence)
2. It causes substantial mortality or disability.
3. It is preventable by certain measure such as immunization
4. The disease that could cause outbreaks such as cholera and meningococcal meningitis as it
affects few numbers of persons within limited time in certain place.
5. It reemerges after successful control measures (TB, cholera, malaria).
6. A disease, which is the target of national, regional or international control, program e.g. polio.
and malaria
7. The measures for the control of this disease should be:

Effective

Available (resources for implementation)
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
Cheap

Accepted by the public.
Indicators of successful surveillance system
1. Completeness: The percentage of reports received in relation to the
targeted number
of reports .
Completeness= the actual number of reports received
X100
The targeted number of reports that should be received
2. Timely: Effective surveillance system provides information on time to make the
response soon enough to minimize the morbidity and mortality.
Timely reports = The number of the reports received on time X 100
The targeted number of reports that should be received
3. Simple: Surveillance system that collects too much useless and complex information is
not efficient
4. Representative: Effective surveillance system should describe the disease in terms of
person, place, and time as well it describes the determinants of the disease.
5. Flexible: Flexibility is important when a new health problem arises (example when
cluster of cases of Aids were found)
6. Hierarchical: The data must be disseminated in a hierarchical manner from the most
peripheral level to the most central level. In this way, health officers at each level should
receive data about the area under their legal authority which can be analyzed and used to
guide the local control activity.
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