<prompt> Which is not an explanation for cyclic trend of diseases -Herd immunity variationsEnvironmental conditionsBuild up of susceptiblesAntigenic variations  <response>: Environmental conditions are responsible for seasonal trend not cyclic trendCyclic trendSome 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 ofMeasles every 2-3 yearsRubella every 6-9 yearsPandemic of influenza every 10-15 years. 