Percentage of the U.S. population that will get the flu, on average, each year: between 5% and 20%.
Number of Americans hospitalized each year because of flu complications: 200,000, on average.
The number of people who die each year from flu-related causes in the U.S.: ranges from 3,000 to 49,000.
The average health costs of the flu for hospitalizations and outpatient doctor visits is over $10 billion.
The typical incubation period for the flu is one to four days. Adults can be contagious from the day before symptoms begin through five to 10 days after the illness starts.
Flu activity usually peaks in January and February.
Number of flu vaccine doses available in the U.S. for the 2014-2015 flu season: Between 151 and 156 million (trivalent variety of vaccine).
The CDC recommends that everyone older than 6 months of age get a flu vaccine as soon as it's available.
The flu vaccine's effectiveness may vary depending on age, health, and immune status and how well scientists identity circulating viruses. In 2014-15, vaccine effectiveness was under 20%.
During 2009-2010, a new and very different flu virus (called H1N1, or swine flu) spread worldwide, causing the first flu pandemic -- global outbreak of disease caused by a new flu virus -- in more than 40 years. It is estimated that the pandemic resulted in more than 12,000 flu-related deaths in the U.S. In contrast to seasonal flu, nearly 90% of the deaths occurred among people younger than age 65.
The 2014-2015 trivalent influenza vaccine is made from the following three viruses:
an A/California/7/2009 (H1N1)pdm09-like virus
an A/Texas/50/2012 (H3N2)-like virus
a B/Massachusetts/2/2012-like virus
Some of the 2014-2015 flu vaccine is quadrivalent vaccine and also protects against an additional B virus (B/Brisbane/60/2008-like virus).
It takes about two weeks after vaccination for an adult to develop antibodies against the flu.
A regular case of the flu typically resolves after three to seven days for the majority of people, although cough and fatigue can persist for more than two weeks.