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Influenza, or flu, is a respiratory infection caused by several flu viruses which are classified as types A, B, and C; type A has a number of subtypes. The flu is not the same as the common cold, which is caused by different viruses such as Rhinoviruses.
Every year in the United States, on average:
Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.
Types of Influenza Viruses
Influenza viruses are classified as type A, B, or C based upon their protein composition. Type A viruses are found in many kinds of animals, including ducks, chickens, pigs, and whales, and also humans. The type B virus widely circulates in humans. Type C has been found in humans, pigs, and dogs and causes mild respiratory infections, but does not spark epidemics.
Type A influenza is the most frightening of the three. It is believed responsible for the global outbreaks of 1918, 1957, and 1968. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin (H) and the neuraminidase (N). There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes. Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses. In the spring of 2009, a new influenza A (H1N1) virus emerged to cause illness in people. This virus was very different from regular human influenza A (H1N1) viruses and the new virus has caused an influenza pandemic.
Flu viruses spread mainly from person to person through coughing or sneezing of people with influenza. Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick.
Symptoms of seasonal flu include:
Complications of flu can include bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.
The single best way to prevent the flu is to get a flu vaccination each year. There are two types of vaccines:
About two weeks after vaccination, antibodies develop that protect against influenza virus infection. Flu vaccines will not protect against flu-like illnesses caused by non-influenza viruses.
Most people ill with influenza will recover without complications. However, any suspected influenza patient who presents with emergency warning signs (for example, difficulty breathing or shortness of breath) or signs of lower respiratory tract illness or worsening illness should seek medical care promptly. The health care provider may prescribe an antiviral medicine such as oseltamivir (trade name Tamiflu®) or zanamivir (trade name Relenza®). Benefits from antiviral treatment is strongest when treatment is started within 48 hours of illness onset.