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