When Flu Occurs
The peak of flu season can occur anywhere from late December through March. In the Southern Hemisphere the season is from June to September. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. Seasons are more likely to be severe if the epidemic originates in China. The Centres for Disease Control monitors circulating flu viruses and their related disease activity and provides influenza reports each week from October through May.
The period when an infected person is contagious depends on the age and health of the person. Studies show that most healthy adults may be able to infect others from 2 days prior to becoming sick and for 10 days after they first develop symptoms. Some young children and people with weakened immune systems may be contagious for longer than a week. Infectivity also varies widely between people, with about one in ten being "hyper-infectors" who are responsible for the majority of influenza transmission. While each flu season is unique, it is estimated that, on average, approximately 5% to 20% of people get the flu each year, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 people die on average per year from the complications of flu.