This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Social & Preventive Medicine
Seasonal trend is due to: 
Select and explain. 
A. Vector variation, B. Environmental factors, C. Change in herd immunity, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Ans. (d) All of the above* Periodic fluctuations:Seasonal trends:1. Is seasonal variation/fluctuation in occurrence of a disease2. Is due to vector variation, environmental factors and change in herd immunity3. Examples:i. Measles (early spring)ii. Upper respiratory infections (winters)iii. Gastrointestinal infections (summers)Cyclical trends:1. Is occurrence of a disease in cycles spread over short periods of time, which may be days, weeks, months or years2. Examples:i. Measles (every 2-3 years)ii. Rubella (every 6-9 years)iii. Influenza pandemics (every 10-15 years)