This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'Which is not an explanation for cyclic trend of diseases - \nSelect and explain. \nA. Herd immunity variations, B. Environmental conditions, C. Build up of susceptibles, D. Antigenic variations', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Environmental conditions are responsible for seasonal trend (not cyclic trend)\n\nCyclic trend\n\nSome diseases occur in cyclic spread over a short period of time which may be week, days, months or years.\nThis is due to naturally occurring variation in herd immunity.\nA build up of susceptible is again required in the herd before there can be another attack.\nIt may be due to antigenic variations as occur in influenza. "\nExamples - Epidemic of\nMeasles\xa0→ every 2-3 years\nRubella\xa0→ every 6-9 years\nPandemic of influenza\xa0→ every 10-15 years.', 'role': 'assistant'}]