Principles of Disease and Epidemiology
Symbiosis – “living together”
 An association between two or more species and one species is typically dependent on the
other
Mutualism -
Commensalism -
Parasitism 
I.
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Infection = multiplication of any parasitic organism within or on the host’s body. If the
infection disrupts the normal functioning of the host then disease occurs
Disease
The production of a change in the normal physiology of the body, the disruption of normal
body functions
Microbial diseases are caused by infection (i.e., growth or replication of the pathogen within
the body) or toxins produced by the pathogen. Note: Infections do not always cause disease
Changes in host may be mild, severe but reversible or irreversible
Pathology is the study of disease and is concerned with the following.
 Etiology - the cause of disease
 Pathogenesis (Pathos = disease, genesis = development) = mechanism by which a pathogen
causes disease (i.e., factors involved in the development of disease). In other words, the
manner in which a disease develops
 Structural and functional changes brought about by disease and the final effects on the body
Pathogen = disease causing (etiologic) agent, a parasite
 pathogenicity is the capacity to produce disease and depends on a number of factors
o ability of pathogen to invade host
o ability of pathogen to multiply in host
o ability of pathogen to avoid host defenses
o the number of infectious organisms that enter the body (Shigella needs only 10 cells
to cause dysentery)
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virulence is the degree or intensity of the disease caused by the pathogen; it is the
quantitative measure of disease. May be expressed as the number of pathogen cells that elicit
a pathogenic response in a host within a given time period
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II.
Pathogenesis is the mechanism by which a pathogen causes disease (i.e., factors involved in
the development of disease).
Normal (indigenous) microbiota
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An adult human is made of 1013 cells and hosts 10 times that many microorganisms on the
skin and mucous membranes and in the passageways of the respiratory, digestive and
reproductive systems.
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Normal microbiota - Organisms that live in or on the host but do not cause disease under
normal conditions. These organisms colonize (establish more or less permanent residence)
the host and are able to multiply. Bacteria, archaea, fungi and protozoa are common
inhabitants of the human body.
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Transient microbiota – persist on the host for short periods of time as long as necessary
conditions are met. Most are not capable of multiplying on the host
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Many factors affect the distribution and composition of the normal microbiota including:
o Nutrients
o chemical and physical factors (e.g., temperature, pH, available O2 and CO2, salinity
and sunlight)
o defenses of the host
o mechanical factors (e.g., chewing action of the teeth, flow of saliva, peristalsis in the
intestine,…)
o “Germfree” animals (devoid of microbiota) may be reared in the laboratory. Animals
colonized by specific strains of microorganisms (gnotobiotic animals) are also created
in the laboratory.
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Normal microbiota can benefit the host in a number of ways
o Microbial antagonism (competitive exclusion) – prevent overgrowth of harmful
microorganisms through competition for resources including nutrients, producing
harmful substances and affecting conditions (e.g., pH and available O2)
o Probiotic – a live microbial culture that is consumed or applied for the purpose of
exerting a beneficial effect on the host
o Prebiotic – a chemical (usually a dietary constituent) that selectively promotes growth
of beneficial microorganisms
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Opportunistic pathogens – do not usually but may cause disease but under certain
circumstances:
o Failure of host’s normal defenses – immunocompromised individuals because of
malnutrition, the presence of another disease, advanced or very young age, treatment
with radiation or immunosuppressive drugs and physical or mental stress
o Introduction of the organisms into unusual body sites
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o Disturbances of the normal microflora – normal microflora interfere with the growth
of pathogens by competing for and depleting resources
III.
Etiology of Disease
How does one go about determining the causative agent (etiologic agent) of a disease?
Koch’s postulates
What are the limitations of Koch’s postulates?
IV.
Classifying Disease
Diseases tend to alter body structures and functions in particular ways and these changes are
indicated by the manifestation of
 Symptoms - changes in body function such as pain and malaise (i.e., vague feeling of body
discomfort) that are subjective and not apparent to an observer.
 Signs – objective changes that can be observed and measured by a health care professional
(e.g., lesions, swelling, fever, paralysis)
 Diagnosis of a disease is made by evaluating signs, symptoms and laboratory tests
A. Kinds of disease
 Infectious – caused by infectious agents such as bacteria, viruses, fungi, protozoa and
helminthes
o Communicable disease is one that is caused by a pathogen that can be spread from
one host to another (contagious diseases are caused by pathogens that are easily
spread from one host to the next)
o Noncommunicable – pathogens are not spread from one host to another
 Infections caused by an individuals normal microflora
 Poisoning following ingestion of preformed toxins (botulism)
 Infection cause by certain organisms found in the environment such as tetanus
or legionellosis
o Emerging infectious diseases – infectious diseases that are new or changing, showing
an increase in the incidence in the recent past or a potential to increase in the near
future. A number of factors contribute to the emergence of a disease, including new
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strains or serovars of a pathogen, widespread use of antimicrobial agents or
pesticides, global warming and weather change, modern transportation, animal
control measures, failure of public health measures, and exposure to pathogens in
regions undergoing ecological changes brought about by natural disaster,
construction, wars or expanding human settlement.
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Noninfectious – caused by any factor other than infectious organisms
B. Occurrence of disease
To understand a disease we need to know something about its occurrence
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Incidence of a disease – the number of people in a population who develop the disease during
a particular time period
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Prevalence - the number of people in a population who have the disease at a specified time
regardless of when the disease started.
Frequency of occurrence is also an important consideration in the understanding of disease
 Sporadic diseases occur only occasionally (e.g., typhoid fever, cholera)
 Endemic diseases are always present in a population (e.g., common cold)
 Epidemic diseases are acquired by many people in a population in a relatively short period of
time (e.g., influenza). A pandemic is an epidemic that is worldwide
C.
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Severity or Duration of disease
Acute disease – develops rapidly but lasts only a short period of time
Chronic disease – develops more slowly and continue or recur over long periods of time
Subacute disease – intermediate between acute and chronic diseases
Latent disease – pathogen remains inactive for a time but becomes active to produce
symptoms of disease
D. Extent of Host involvment
Infections are classified by the extent to which the host’s body is affected
 Local infection – infectious microbes are limited to a relatively small area of the body
 Systemic infections are general infections in which the pathogen or their products are spread
throughout the body
 Focal infection – pathogens from a local infection enter the blood or lymphatic vessel and
spread to other parts of the body where they are confined
Sepsis – toxic inflammatory condition arising from the spread of microbes from a local infection
Septicemia – often called blood poisoning – systemic infection arising from multiplication of the
pathogen in the blood. The presence of bacteria in the blood is known as a bacteremia. Toxemia
refers to the presence of toxins in the blood
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The state of host resistance also determines the extent of infections
 Primary infections are acute infections caused by the initial illness.
 Secondary infection is one caused by opportunistic pathogens due to the weakened state of
the host after the primary infection
 Subclinical infections do not cause noticeable illness
E. Factors affecting the occurrence of disease
i. Predisposing factors – make the body more susceptible to disease
e.g., gender, genetic background, climate & weather, indequate nutrition, age, environment,
habits, lifestyle, occupation, preexisting illness, chemotherapy, emotional disturbances, stress
ii. Reservoirs of infection – source of the pathogen – can be biotic or abiotic
 Carrier – harbour the pathogen without signs of infection and serve as important living
reservoirs of infection
 Animals may serve as reservoirs for pathogens that infect humans. Infectious diseases that
are caused by pathogens that can be transmitted from wild or domestic animals to humans are
known as zoonoses
iii. Transmission of disease
The pathogen is transmitted from the reservoir of infection to the susceptible host by three
principal routes
 Contact
o Direct contact (also known as person to person transmission)
o Indirect contact – pathogen spread from reservoir to host by an inanimate object
o Droplet transmission – pathogen spread across short distances (up to 1 m) in airborne
droplets discharged from an infected individual
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Vehicle transmission – pathogens are spread by a medium such as water, food or air
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Vectors – animals that carry pathogens from host to host. Arthropods are the most important
vectors of disease. The spread may through mechanical transmission (passive transport) or
biological transmission (an active process often involving the biology of the vector, i.e.,
vector biting and blood feeding on host)
Nosocomial infections – hospital acquired infections result from a combination of factors
including the weakened state of the host, the microorganisms found in the hospital environment
and the chain of transmission in the hospital
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F. Development of disease
There is a definite sequence of events that occurs during infection and disease, once the
microorganism overcomes the host defenses.
 Incubation period – interval between initial infection and the first appearance of signs and
symptoms. This period depends on a number of factors including the pathogen involved, its
virulence, the number of infecting microorganisms and the resistance of the host
 Prodromal period – follows the incubation period in some diseases. It is a relatively short
period that is characterized by mild symptoms of the disease including general aches and
malaise
 Period of illness – The period when the disease is most severe with the host exhibiting overt
signs and symptoms. Generally the hosts immune response and other defense mechanisms
overcome the pathogen and the period of illness ends
 Period of decline – the signs and symptoms subside. Host is susceptible to secondary
infections at this time
 Period of convalescence – Host regains strength and body returns to its prediseased state
V.
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Epidemiology
Epidemiology is the study of when and where diseases occur and how pathogens are
transmitted in populations.
Epidemiology examines not only the etiology of a disease but also other factors and patterns
concerning the hosts infecting, including age, sex, occupation, personal habits,
socioeconomic status, history of immunization, common histories of infected individuals, site
where individual can in contact with the pathogen, and period of the year that the disease
occurs.
Epidemiologists are also concerned with various methods for controlling disease, such as
chemotherapy, immunization, hygiene, control of reservoirs, water and waste water
treatment, food preservation, changes in personal habits, food preparation and cooking, host
nutrition and screening of blood products and transplanted organs
Morbidity rate – the number of people affected by a disease in a given period of time in relation
to the total population
Mortality rate – the number of deaths resulting from a disease in a population in a given period
of time in relation to the total population
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