Types of epidemics from park
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Park's Textbook is in the library. Let me read the epidemic section directly.
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Now let me also look for the section specifically on "types of epidemics" (common source, propagated, etc.) which Park covers under epidemic investigation.
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Found the dedicated section. Let me read it fully.
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Now let me fetch the epidemic curve image and check for C. Slow epidemics content.
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I now have all the content needed. Here is the complete, detailed answer directly from Park's Textbook:
Types of Epidemics - Park's Textbook of Preventive and Social Medicine
Park distinguishes three major types of epidemics:
A. Common-Source Epidemics
In these epidemics, cases arise from exposure to a single contaminated source. There are two sub-types:
(a) Single Exposure / "Point-Source" Epidemics
- Exposure to the disease agent is brief and essentially simultaneous
- All resultant cases develop within one incubation period of the disease
- Classic example: food poisoning outbreak
Features of the epidemic curve:
- Rises and falls rapidly, with no secondary waves
- Epidemic is explosive - cases cluster within a narrow time interval
- The "median incubation period" (time for 50% of cases to appear after exposure) can be calculated from the curve
Below is the classic point-source epidemic curve (Fig. 4 from Park):
Note: Common-source epidemics can result not only from infectious agents but also from environmental contamination by industrial chemicals (e.g., Bhopal gas tragedy, Minamata disease from mercury-contaminated fish).
(b) Common-Source, Continuous or Repeated Exposure Epidemics
- The exposure from the same source is prolonged, repeated, or intermittent
- No explosive rise; the epidemic is more extended or irregular
- Examples:
- A contaminated water well supplying a community over weeks
- A nationally distributed contaminated food or vaccine batch
- The Legionnaire's disease outbreak in Philadelphia, 1976 (continued beyond one incubation period, no secondary person-to-person cases)
A variation: an epidemic may begin as common-source and then continue as propagated - e.g., water-borne cholera reaches a sharp peak but tails off gradually.
B. Propagated Epidemics
- Most often of infectious origin, resulting from person-to-person (or vector/animal) transmission
- The epidemic shows a gradual rise and tails off over a much longer period
- Transmission continues until susceptibles are depleted or no longer exposed
- Speed of spread depends on: herd immunity, opportunities for contact, and secondary attack rate
- More likely where large numbers of susceptibles are aggregated, or where there is a regular supply of new susceptibles (newborns, immigrants) - lowering herd immunity
- Examples: hepatitis A, polio
Sub-types of propagated epidemics:
| Sub-type | Mechanism |
|---|---|
| (a) Person-to-person | Direct human transmission |
| (b) Arthropod vector | Via mosquito, tick, etc. |
| (c) Animal reservoir | From an animal source |
C. Slow (Modern) Epidemics
- These are epidemics of non-communicable, chronic diseases such as cancer, cardiovascular diseases, obesity, etc.
- Their existence is not easily recognized unless a comparison is made with previous experience (unlike the obvious onset of cholera or food poisoning outbreaks)
- They evolve slowly over years or decades
Related Terminology (from Park)
| Term | Meaning |
|---|---|
| Endemic | Constant presence of a disease in a given geographic area at usual/expected frequency |
| Hyperendemic | Constantly present at high incidence, affecting all age groups equally |
| Holoendemic | High infection beginning early in life (e.g., malaria) |
| Pandemic | Epidemic crossing international boundaries affecting a large number; e.g., influenza pandemics, cholera |
| Sporadic | Irregular, infrequent, geographically scattered cases; e.g., tetanus, herpes zoster |
| Exotic | Diseases imported into a country where they do not otherwise occur |
| Epizootic | Epidemic in an animal population; e.g., anthrax, rabies, brucellosis |
| Epornithic | Epidemic in a bird population |
| Enzootic | Endemic in animals |
Source: Park's Textbook of Preventive and Social Medicine, Chapter on Epidemiology - Types of Epidemics (pp. 76-77)
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