The exact timing and length of a flu season varies from year to year, but generally runs from October to May, according to the Centers for Disease Control and Prevention. The highest amount of flu activity occurs between the months of December and February.
Researchers believe that flu activity is highest during the winter because the temperatures preserve the virus, notes The New York Times. Flu viruses become more stable and more transmissible when the air is cold and dry. Dry air allows the viral particles to remain airborne and travel from host to host. Humid air weighs down the particles, causing them to fall from the air.
The seasonal flu is a respiratory illness that affects 5 to 20 percent of Americans each year, as of 2015. The illness is contagious in adults up to one day before any signs or symptoms are observed. Adults remain contagious five to seven days after the onset of symptoms. Individuals with weakened or compromised immune systems, such as young children and the elderly, can remain symptomatic and contagious for an extended period.