Flu season

Flu season is an annually recurring time period characterized by the prevalence of outbreaks of influenza (flu). 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.
The 2012–13 flu season was particularly harsh in the United States, where the majority of states were reporting high rates of influenza-like illness. The Centers for Disease Control reported that the available flu vaccine was 60% effective. It further recommended that all persons over age 6 months get the vaccine. The 2013-14 flu season is currently in progress and is above epidemic levels in many countries, including Canada, the United States, Greece and Finland.
Cause
Three virus families...