Seasonal trend is due to:

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)