Which is not an explanation for cyclic trend of diseases -

Environmental conditions are responsible for seasonal trend (not cyclic trend) Cyclic trend Some diseases occur in cyclic spread over a short period of time which may be week, days, months or years. This is due to naturally occurring variation in herd immunity. A build up of susceptible is again required in the herd before there can be another attack. It may be due to antigenic variations as occur in influenza. " Examples - Epidemic of Measles → every 2-3 years Rubella → every 6-9 years Pandemic of influenza → every 10-15 years.