Epidemiological Characteristics of Infectious Diseases
Prepared by: Dr. Hala A. Abed
 To describe any infectious disease, you should comment on:
 Definition of the disease( bacterial ,viral, zoonotic
……….etc).
 Magnitude of the problem.
 Epidemiology of the disease:
 Agent.
 Reservoir: man only, animal only or man& animal.
 Mode of transmission.
 Incubation period.
 Susceptibility:
*Distribution of the disease according to time , place person.
* Immunity: natural( active& passive) and acquired (active&
passive).
* Herd Immunity.
 Diagnosis : - Clinical picture, complication and
investigation.
 Prevention
 Control
1.
Incubation Period(IP):

Definition: It is the time between exposure to an infectious
agent and the onset of symptoms or signs of infection.

Each infectious disease has a typical IP that requires
multiplication of the infectious agent to a threshold necessary to
produce symptoms or laboratory evidence of infection.
 Causes of variability of IP:
 The dose or inoculum of the infectious agent.
 The route of inoculation.
 The rate of replication of the organism.
 Host factors(e.g. resistance, immunity, co-morbidity).
 Duration of IP:
 Shortest IP: few hours as in staphylococcal food poisoning.
 Diseases with short IP period 1-5 days (GISS + DC)are:
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(Gonorrhea,Influenza,Shigella, Scarlet fever,Diphteria & Cerebrospinal
meningitis).
 Diseases with medium IP 6-12 days (3PRAM) are:
(Pertussis, Plague, Poliomyelitis, Relapsing fever, Anthrax & measles).
 All other diseases have IP period 2-3 weeks except:
-Cholera 5days.
-Plague 6days.
-yellow fever 6days.
 IP of parasitic diseases is 4 weeks.
 Diseases with long IP are:
-Hepatitis A Virus 1.5 months.
- Hepatitis B Virus 2-6 months.
 Very long IP:
-Leprosy 2-5years.
-AIDS 6m- 10years.

Epidemiological importance of IP:
Control of infectious diseases to trace source of infection.
Control of contact of cases either by surveillance for
maximum IP.

International measures for qurantinable diseases.

Specific protection of exposed individuals as in:
Cases of exposure to small pox early in IP person could be
vaccinated as the immunity is produced after 8 days while IP is 14
days.


N.B Extrinsic incubation period: This the period taken by the
infectious agent outside the human body until it becomes infective
again to a new individual.
 Latent period: The time that elapses between the entry of the
agent in the human body to the point when the shedding of the
organism starts.
 Period of communicability: This the duration for which the host
sheds the organism.
 Generation time: The duration between the entry of the
infectious agent into the body to the peak of infectivity of the
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host, it is equal to( latent period + period of maximum
communicability).
2.
Biological Characteristics
of the Organism

Infectivity:
 Definition: It is the ability of the agent to cause infection in
a susceptible host.
 Measurement: The proportion of susceptible individuals
who develop infection after exposure ( secondary attack
rate) is a measure of the infectivity of an organism.
 Pathogenicity:
 Definition: It is the ability of the organism to induce disease.
 Measurement: The proportion of individuals with asymptomatic
or clinically inapparent infections is the measure of the
pathogenicity of the organism.
 Virulence:
 Definition: it is the severity of the disease after infection
occurs.
 Measurement: It can be measured by case fatality rate or the
proportion of clinical cases that develop sever disease.
 Immunogenecity:
 Definition: It is the ability of the organism to produce an
immune response after infection.
 Advantage of Immunogenecity:
- Some organisms such are measles, yellow fever lead to solid
immunity.
- Studies of the antigens that produce protective immunity after
natural infections have led to development of effective
vaccines.
- Antibodies may occur that are markers of a previous or current
infections.
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 Disadvantage of Immunogenecity:
-Immune response that is provoked by some agents may not be
protective from future infections.
- Sometimes the immune response may be harmful to the host
e.g. Rheumatic fever.
3.
Infectious process
 Infection: the entry and development or multiplication of an
infectious agent in the body of man or animals.
 Infestation: the presence of living infectious agents on the
exterior surface of the body(e.g. scabies).
 Contamination: The presence of living infectious agents
upon articles( soiled articles).
 Endemic:……………………………………
 Epidemic:…………………………………..
 Outbreak:………………………………….
 Pandemic:………………………………….
 Sporadic:……………………………………
 Non-infectious diseases:………………..
4.
Classification of infectious
diseases according to their
reservoir:
a) Man only:
 Cases: Varicella- H.Z-Pertussis-measles-Epidemic typhusInfective conjunctivitis-scabies- Syphilis-GonorrheaLeprosy.
 Cases& Carriers: Diphteria-Mumps-RubellaStrept.Pharingitis-Meningocoocal meningitis-EntericaPoliomylitis-Shigellosis-Cholera-Viral Hepatitis.
b) Animal only: Brucellosis- Botulism-Plague-Q feverAnthrax-Rabies- Rift valley fever- Foot and mouth
disease- Encephalitis.
c) Man and Animal: T.B- Influenza- Infectious food
poisoning – Yellow fever- Tetanus- Gas gangrene.
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** Infectious diseases with four types of carriers: EntericaPoliomyelitis- Cholera.
** Infectious diseases with highest incidence at 5-15years
old: Mumps- Rheumatic- Pertussis- M.Meningitis- Influenza- HAV.
** Infectious diseases produce neurotoxin: Tetanus- BotulismShigella shiga.
Specific prevention of
infectious diseases:
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a) Seroprophylaxis only: Botulism only.
b) Active immunization only: Enterica-Yellow feverPneumococcal.pn-H. Influenza b-Epidemic typhus.
c) Immunization ,both forms: Diphtheria, Measles-MumpsRubella, Varicella, Poliomyelitis- Hepatitis A , B-Rabies.
d) Chemoprophylaxis: Strept. Pharyng- Puerperal sepsisSyphilis- Gonorrhea.
e) Seroprophylaxis & Chemoprophylaxis: Gas gangrene
only.
f) Active immunization & Chemoprophylaxis: InfluenzaM.Meningitis-T.B-Cholera-Plague- Leprosy.
g) Immunization ,both forms& Chemoprophylaxis:
Pertussis-Tetanus.




** Protective value of vaccines:
Absolute: Yellow fever- Varicella- Rabies vaccines.
Solid : M.M.R- DT toxoid- HBV (99%).
High protection: BCG- Sabin vaccine- Pertussis vaccine (8090%).
Moderate: TAB vaccine- Cholera vaccine (40-60%).
** Protective period of vaccines:
 Short period for some months: Cholera – Plague vaccines.
 About 3 years: TAB vaccine.
 3-5 years: DPT- Tetanus toxoid.
 5 or more years: BCG, Epidemic typhus vaccine.
 Life time protection: Yellow fever & MMR vaccines.
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