<prompt> All are true regarding point-source epidemic except?All cases occur abruptly and simultaneouslyChildren are most commonly affectedOccurs within a specified periodNo secondary waves  <response>: Ans. is b i.e., Children are most commonly involved o The main aim of descriptive epidemiology is to study the distribution of disease in time, place and person. Time distribution o There are three kinds of time trends or fluctuations in disease occurence :? A. Sho term fluctuations epidemics B. Secular trends or long term fluctuations C. Periodic fluctuation Sho term fluctuation o The best known sho - term fluctuation in the occurence of a disease is an epidemic. o There are three major types of epidemics ? 1 Common - source epidemics a Single exposure or point source epidemics b Continuous or multiple exposure epidemics 2 Propagated epidemics 3 Slow modern epidemics Common source epidemics q As name suggests, the source of disease agent is common for all cases. q It is of two types. a Single exposure or point source epidemics o The exposure to the disease agent is brief and essentially simultaneous -- single exposure. Because disease agent enters into all exposed persons at same time -- the resultant cases all develop within one incubation period of the disease. The epidemic curve rises and falls rapidly. Because exposure is single simultaneous with no fuher exposure --- No secondary wave no secondary case. Epidemic tends to be explosive, there is clustering of cases within a narrow interval of time. o Point source epidemics are mostly not always due to exposure to an infectious agent. Examples -- Food poisoning, Bhopal gas tragedy, Minamata disease in Japan, Chernobyl tragedy. b Continous or multiple exposure epidemics o Exposure from the common same source is prolonged - continous, repeated or intermittent. o Exposure is not at the same time or place. o For examle, a prostitute may be a common source in a gonorrhea outbreak, but since she will infect her clients over a period of time there may be no explosive rise in number of case. Because disease agent enters into exposed person at different time -- outbreaks continues over more than one incubation period. o Example -- Outbreak of legionnaires disease respiratory illness in 1976 in Philadelphia USA. 