Which is not an explanation for cyclic trend of diseases -: (A) Herd immunity variations, (B) Environmental conditions, (C) Build up of susceptibles, (D) Antigenic variations
Answer is B. 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.