<prompt> Seasonal trend is due to:Vector variationEnvironmental factorsChange in herd immunityAll of the above  <response>: Ans. d All of the above Periodic fluctuations:Seasonal trends:1. Is seasonal variationfluctuation in occurrence of a disease2. Is due to vector variation, environmental factors and change in herd immunity3. Examples:i. Measles early springii. Upper respiratory infections wintersiii. Gastrointestinal infections summersCyclical 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 yearsii. Rubella every 6-9 yearsiii. Influenza pandemics every 10-15 years 