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What Is the Flu?

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Who's at greatest risk for flu complications?

While anyone can get flu, infants, the elderly, pregnant women, and people with chronic ailments such as diabetes, heart disease, lung disease, and HIV are at highest risk for flu complications. Despite advances in flu prevention and treatment, the CDC estimates that deaths related to influenza range from 3,000 to 49,000 deaths in the United States each year.

Specific strains of flu can be prevented by a flu vaccine, either a flu shot or nasal spray flu vaccine. In addition, antiviral medications are available to prevent flu. These drugs may help reduce the severity and the duration of flu and are best used within the first 48 hours of the appearance of flu symptoms.

For in-depth information, see WebMD's Flu Complications.

Are there different types of flu viruses?

Researchers divide flu viruses into three general categories: types A, B, and C. All three types can mutate, or change into new strains, and type A influenza mutates often, yielding new strains of the virus every few years. This means that you can never develop a permanent immunity to influenza. Even if you develop antibodies against a flu virus one year, those antibodies are unlikely to protect you against a new strain of the flu virus the next year.

Type A mutations are responsible for major flu epidemics every few years. Type B is less common and generally results in milder cases of flu. However, major flu epidemics can occur with type B every three to five years.

Type C causes infection but does not cause typical flu symptoms. Both influenza A and B have been linked to the development of Reye's syndrome, a potentially fatal complication that usually affects children and teens under age 18. Widespread outbreaks of Reye's syndrome have occurred with influenza type B and also with chickenpox, but other viruses have been implicated. The risk of Reye's syndrome is increased when taking aspirin, so children should not take aspirin.

Most influenza viruses that infect humans seem to originate in parts of Asia, where close contact between livestock and people creates a hospitable environment for mutation and transmission of viruses. Swine, or pigs, can catch both avian (meaning from birds, such as poultry) and human forms of a virus and act as hosts for these different viral strains to meet and mutate into new forms. The swine then transmit the new form of the virus to people in the same way in which people infect each other -- by transmitting viruses through droplets in the air that people breathe in.

For in-depth information, see WebMD's Types of Flu.

What is avian or bird flu?

Bird flu, or avian influenza, is an infectious disease of birds caused by type A strains of the influenza virus. Bird flu epidemics have occurred worldwide. 

Bird flu is a leading contender to be the next pandemic flu bug because it has caused an unprecedented epidemic in poultry and wild birds across Asia and Eastern Europe. Still, no one knows for sure whether this will cause the next human flu pandemic.

For in-depth information, see WebMD's Understanding Avian or Bird Flu.

WebMD Medical Reference

Reviewed by David T. Derrer, MD on November 21, 2013
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