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.
Herd immunity variations
Environmental conditions
Build up of susceptibles
Antigenic variations