This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'True about single exposure, common vector outbreak: \nSelect and explain. \nA. Explosive onset, B. Secondary waves occur, C. Cases occur in varying incubation periods, D. Depends on herd immunity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Explosive onsetRef: Park, 24th edition, page 69; Leon Gordis, Epidemiology, 5th edition, page 29; Principles and Practice of Community Medicine by DrAsma Rahim, 2nd edition, page 107Explanation:Point Source EpidemicHere simultaneous brief exposure to the disease agent takes place. The resultant cases occur within one incubation period.Features (as shown in Figure)Epidemic curve rises and falls rapidly.There are no secondary waves.Epidemic tends to be explosive.Clustering of cases within a narrow interval of time.All cases occur within one incubation period.Examples:Bhopal Gas Tragedy-1984.Chernobyl explosion-1986.Minamata disease-due to methyl mercury poisoning.Continuous Exposure EpidemicSharp rise in number of cases and fall in number of cases interrupted by "secondary waves/peaks", e.g. Contaminated well in a village, prostitute in STD outbreak.Propagated EpidemicsGradual rise and gradual fall over a long time.The rate of spread depends upon:Herd immunity.Contact.Secondary attack rate.E.g. Hepatitis A and polio epidemics.', 'role': 'assistant'}]