Ans: a (Point source epidemic) Ref: Park, 19th ed, p. 60There are three kinds of time trends or fluctuation in disease occurrence1. Short term fluctuation2. Periodic fluctuation3. Long term or secular trendsShort term fluctuations-Best known example is an epidemic.Types of epidemics:A. Common -source epidemicsa) Single exposure or point source epidemicsb) Continuous or multiple exposure epidemicsB. Propagated epidemicsa) Person-to-personb) Arthropod vectorc) Animal reservoirC. Slow or modem epidemicCommon-source epidemics:1. S ingle exposure or point source epidemics-The exposure to the disease agent is brief and simultaneous, the resultant cases all develop within one incubation period of the disease. The main features of a point source epidemic are:i. The epidemic curve rises and falls rapidly, with no secondary wavesii. The epidemic tends to be explosive, there is clustering of cases within a narrow interval of timeiii. All cases develop within one incubation period of the disease.Examples -Bhopal gas tragedy in India, Mina Mata disease in Japan, food poisoning2. Common -source, continuous or repeated exposure epidemicsExposure from the same source may be prolonged, or repeated. The resulting epidemics tend to be more extensive and irregular. Diseases tend to occur beyond the range of one incubation period. No evidence of secondary cases among persons who had contact with ill persons. E.g. A prostitute being a common source in gonorrhea outbreak.Propagated Epidemics:The epidemic usually shows a gradual rise and tails off over a much longer period of time. Transmission continues until the number of susceptible is depleted. E.g., polio, hepatitis APeriodic fluctuations:a) Seasonal trend-upper respiratory tract infections in winter, gastrointestinal infections in summer.b) Cyclic trend-diseases may occur in cycles spread over a short period of time. E.g., Measles in pre vaccination era appeared every 2-3 years, rubella every 6-9 years, Influenza pandemic every 7-10 years.Long term or secular trends - Changes in occurrence of disease over a long period of time, over years or decades.e.g., coronary artery disease, lung cancer, diabetes.Endemic - it refers to the constant presence of a disease or an infectious agent within a given population group or geographic area without importation from outside.