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Flu is most common during cold months (between December and March in the northern hemisphere).

Every year in the United States:^{1 2}

• About 1 in 5 adults catch the flu
• About 1 in 20 children get the flu
• About 200,000 people are treated in hospitals
• About 30,000 people die of the flu. About 90 percent of the people who die because of the flu are aged over 65.

Most people who go to the hospital because of flu have pneumonia or another complication.^{2 3} Children under 1 year are also more likely to need to go to the hospital with flu.¹ About 92 children under 5 die from flu in the United States each year.⁴

Flu viruses change over time. This is called mutation. Normally, when your body fights off a virus, you don't get sick if you come into contact with the same virus again. Your immune system remembers how to kill the virus, and you don't get sick. But if the flu virus changes, your body doesn't know how to fight off the new strain.

When there is a new strain of flu that very few people are immune to and it affects people all over the world, it's called a pandemic. Three flu pandemics happened in the 20th century. The last one happened in 1968.²

Some scientists think that a kind of flu that affects birds could mutate or mix with human flu. If this happened, there's a chance it could cause a flu pandemic. To read more, see Avian flu.

Just in case this happens, the government is stockpiling antiviral drugs. But it's difficult to know how well these drugs would work against a new type of flu. When pandemics happen, scientists try to make vaccines for new types (strains) of flu as quickly as possible.⁵

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.