Flu season is a regularly re-occurring time period characterized by the prevalence of outbreaks of influenza. The season occurs during the cold half of the year in each hemisphere. Influenza activity can sometimes be predicted and even tracked geographically. While the beginning of major flu activity in each season varies by location, in any specific location these minor epidemics usually take about 3 weeks to peak and another 3 weeks to significantly diminish.Individual cases of the flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.
Cause
Three virus families, Influenzavirus A, Influenzavirus B, and Influenzavirus C are the main infective agents that cause influenza. During periods of cooler temperature, influenza cases increase roughly tenfold or more, resulting in the flu season. Despite higher prevalence of disease cases during the season, these viruses are transmitted amongst people all year round. They do not disappear and reappear.
Each annual flu season is normally associated with a major influenzavirus subtype. The associated subtype changes each year, due to mutational changes amongst viral populations, as well as the development of immunological resistance to last year's strain from previous infection and vaccination.
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