<prompt> False regarding a propagated epidemic is ?No secondary wavesSpread depends on herd immunityPerson to person transmission occursSlow rise  <response>: Ans. is a i.e., No secondary wave Propagated epidemics o Propagated epidemic is most often of infectious origin. o It results from person to person transmission. Source of disease agent is not common to all cases. Every new case becomes the source for others. The epidemic usually shows a gradual rise and tails off over a much longer period of time. o The speed of spread depends upon herd immunity, oppounities for contact and secondary attack rate. o Propagated epidemics are more likely to occur where large number of susceptibles are aggregated, or where there is a regular supply of new susceptible individuals e.g. bih, immigrants lowering herd immunity. o Transmission continues until the number of susceptibles is depleted or susceptible individuals are no longer exposed to infected persons or intermediary vectors. o Epidemic continues over more than one incubaton period. Secondary waves are seen. o Examples --- epidemic of polio and Hepatitis A. Also know o A typical type of epidemic is epidemic of cholera. o It stas as common source epidemic --- most of the initial cases acquire the disease from a common source e.g. a well with contaminated water. o Then it continues as a propagated epidemic --- these cases become the source for the other persons in their close vicinity. So it has a sharp peak but tails of gradually over a longer period of time. 