Here's a rundown of some important statistics based on the best available data.
5% to 20% -- Percentage of the U.S. population that will get the flu, on average, each year.
200,000 -- Average number of Americans hospitalized each year because of problems with the illness.
3,000 to 49,000 -- Number of people who die each year from flu-related causes in the U.S.
$10 billion+ -- Average costs of hospitalizations and outpatient doctor visits related to the flu.
1 to 4 days -- Typical time it takes for symptoms to show up once you've caught the virus. Adults can be contagious from the day before symptoms begin through 5 to 10 days after the illness starts.
December to February -- Peak flu season in the U.S.
171 million to 179 million -- Number of flu vaccine doses expected to be available in the U.S. for the 2015-2016 flu season.
6 months -- The youngest age for which the CDC recommends a flu shot.
Swine flu -- A new type that spread worldwide during 2009-2010, causing the first flu pandemic -- global outbreak of disease -- caused by a new flu virus in more than 40 years. It's estimated that the pandemic caused more than 12,000 flu-related deaths in the U.S. In contrast to seasonal flu, nearly 90% of the deaths were of people younger than 65.
- A/California/7/2009 (H1N1)pdm09-like virus
- A/Switzerland/9715293/2013 (H3N2)-like virus
- B/Phuket/3073/2013-like virus (B/Yamagata lineage)
Some of the 2015-2016 flu vaccine is "quadrivalent" and protects against an additional B virus (B/Brisbane/60/2008-like virus).
2 weeks -- Time it takes after vaccination for an adult to develop disease-fighting antibodies against the flu.