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Flu season is a term used to describe the regular outbreak in flu cases during the cold half of the year. Flu 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.[1] 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. The annual flu (also called "seasonal flu" or "human flu") in the U.S. "results in approximately 36,000 deaths and more than 200,000 hospitalizations each year. In addition to this human toll, influenza is annually responsible for a total cost of over $10 billion in the U.S." [2] The garden variety flu that comes around every year is caused by Influenzavirus A, Influenzavirus B, or Influenzavirus C and are also known as human flu virus strains which is to say it has made genetic changes to adapt to its human hosts. It passes from human to human all year round and never goes away completely. When it is cold, infection from "human flu" increases roughly tenfold or more. Different strains of flu virus circulate in different years as it is constantly mutating. The influenza vaccine for the 2005 - 2006 flu season contains proteins from the coat of two subtypes of species A and from species B. Species B and C don't have subtypes. It remains unclear why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. Another is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus may also linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures. Increased travel and visitation due to the holiday season may also play a role.[3]
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