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The flu (influenza) is a highly contagious respiratory disease caused by a virus. In temperate areas, disease outbreaks are often seasonal. They tend to occur in the winter and typically last two to three months. In the northern hemisphere, the annual peak of the disease is usually between December and March. In tropical regions, the flu can occur throughout the year.

The U.S. Centers for Disease Control and Prevention (CDC) collects data on flu distribution and its impact. Each year, it is believed to infect from 5 to 20 percent of the United States population. Of those infected, more than 200,000 people are hospitalized and 36,000 die from the flu or its complications.

The flu is most likely to infect children but other populations feel the most severe impact. The majority of flu-related deaths occur in people older than 65 years of age. Complications may also be serious in children younger than two years old and patients with underlying conditions, including malnutrition, asthma and other lung diseases.

Like all viruses, the flu viruses are tiny particles that must infect cells of other organisms to reproduce. When viewed through an electron microscope, it appears similar to a ball with spikes coming from the surface. The ball is made largely of a mix of proteins that form the virus membrane and contains the genetic material (RNA or ribonucleic acid) necessary for the virus to reproduce. The spikes on the virus are surface proteins embedded in the membrane of the ball.

Viruses, including the flu, must invade cells of other organisms to reproduce. The body’s immune system responds by producing antibodies. The antibodies recognize the viruses by the surface proteins and therefore know which particles to remove without damaging normal body cells. These antibodies stay in the body to prevent future infection.

Flu viruses are able to change or mutate their structure over time to bypass the antibodies. This allows outbreaks to occur each year and results from two processes. Through antigenic drift, the surface proteins of the virus change structurally and existing antibodies do not recognize the particles as infecting agents. The immune system must then create new antibodies to fight the infection of these changed viruses.

Antigenic shifts are more significant changes. They occur when viruses that typically infect different animal species are present in the same organism and trade genetic material. There is no natural immunity to the resulting virus and it can quickly cause widespread disease. This has happened in the past in 1957, 1968 and 1977, and is believed to be responsible for the 1918 flu pandemic that caused more than 20 million fatalities.

Past epidemics are believed to have resulted from flu strains mixing in pigs. Swine can host viruses from both birds and people, providing opportunities for antigenic shifts. Many scientists believe it could also occur in people now that some forms of bird flu are also now known to infect humans.