Increases and or decreases in the frequency of a disease over years or decades are known as Cyclic trends.
Diseases occur in cycles spread over short periods of time which may be days, weeks, months or even years. For example, in pre-vaccination period Measles appeared in cycles over a period of 2 to 3 years whereas Rubella occurred every 6 to 9 years as a result of natural variation in herd immunity.
Non-infectious conditions like automobile accidents may however increase considerably during the rains, whereas, Influenza pandemics tend to occur between an interval of 7 to 10 years due to antigenic variation.
A good understanding about the Cyclic trends of diseases helps us to deal with them in a better way and defend ourselves.
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