Lesson 2: Epidemiology of Communicable Diseases

Lesson 2: Epidemiology of Communicable Diseases

Basic concepts in the epidemiology of infectious disease
Identification
Infections can be identified by their clinical features, epidemiology and the use of appropriate
laboratory procedures.
Epidemiological triangle
The traditional model of infectious disease causation is the epidemiological triangle. It has three components: an external agent, a susceptible host and environmental factors that bring the host and the agent together. The agent is the organism (virus, rickettsia, bacterium, fungus, prion, etc.) that produces the infection. Host factors influence an individual’s exposure, susceptibility or response to a causative agent. Age, sex, socio‐economic status, ethnicity and lifestyle factors such as smoking, sexual behaviour and diet are among the host factors that affect a person’s likelihood of exposure, while age, genetic makeup, nutritional and immunological status, other disease states and psychological makeup influence susceptibility and response to an agent. Environmental factors are extrinsic factors that affect the agent and the opportunity for exposure. These include geology, climate, physical surroundings, biological factors (such as insect vectors), socio‐economic
factors such as crowding and sanitation and the availability of health services.
Natural history of disease
This refers to the progress of a disease in an individual over time without intervention. Following exposure to an infectious agent there is a period of subclinical or inapparent pathological changes, which ends with the onset of symptoms. This period is known as the incubation period. For a given infectious disease, the incubation period has a range and a median and mean value. For hepatitis A, the range is two to six weeks with a mean of three weeks. During the incubation period,
pathological changes may be detectable with laboratory or other tests. Most screening programs
attempt to identify the disease process during this early phase of its natural history, since early intervention may be more effective than treatment at a later stage. The onset of symptoms marks the transition from the subclinical to the clinical phase. Most diagnoses are made during this stage.

 

 

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