Types of epidemics from park

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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):

Point-source epidemic curve - rapid rise and fall after a single exposure

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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