Seasonal influenza, also known simply as the flu, infects 5 to 20 percent of the population each year in the United States, according to Flu.gov. Influenza usually starts in late autumn and is widespread during the winter months into early spring. Flu viruses mutate each year, so having the flu one year doesn't mean a person won't get the flu the next year.
Types
Three types of influenza virus--A, B and C--can infect people. Of the three, A and B most commonly occur. Because C occurs less frequently, it isn't included in the annual flu vaccine. Type A is further divided into a number of subtypes, including H1N1 and H3N2, according to the World Health Organization.
Spread
Flu spreads through contact with respiratory droplets containing the virus. People with the flu are contagious for a day before and up to seven days after they first show symptoms. Children and people with suppressed immune systems may remain contagious for a longer time.
Symptoms
Symptoms of the flu often include fever, chills, muscle aches and pains, sore throat, cough, headache, extreme fatigue and runny or stuffy nose. Most people recover within a week.
Complications
Around 36,000 people died each year from the flu during the 1990s, the Centers for Disease Control and Prevention says. Ninety percent of people who die from the flu are over age 65. Older people, children, pregnant women and people with disease such as asthma, heart disease and diabetes more often develop complications of the flu, such as pneumonia, ear and sinus infections or dehydration. People with compromised immune systems and chronic health conditions also develop complications more frequently.
Susceptibility
Each year, a flu vaccine is available in two forms--an inactive or killed virus injection and a live virus nasal spray, sometimes called a live attenuated influenza vaccine, or LAIV. The vaccine incorporates the strains that experts surmise will be most prevalent during the upcoming year. The killed virus is given to healthy people as well as those with chronic health conditions. Only healthy people between ages 2 and 49 should receive the LAIV, the CDC states. Around 70 to 90 percent of flu cases can be prevented by the vaccine in healthy people, as well as around 60 percent of cases and 80 percent of deaths in high-risk people.
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