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Bird flu is a viral disease that can be transmitted from birds to people. Since 2003, more than 350 people have been infected with the virus and more than half have died. To date, most cases are caused by close contact with poultry. In addition, there are limited instances of transmission from wild birds to domestic poultry and from person to person. Some experts believe the virus’ method of transmission could change, allowing the disease to rapidly infect a large percentage of the global population.

Bird flu is present in many wild birds without the bird showing signs of infection. The virus is found in the animal's saliva, nasal secretions and feces. It can be spread through direct contact with an infected animal or through contaminated surfaces (e.g., cages, dirt) or materials (e.g., water, feed).

The first indication of disease is often seen in domesticated birds (including chickens, turkeys, geese and ducks). Symptoms in poultry may range from minor (e.g., ruffled feathers, reduced egg production, mild respiratory effects) to severe (e.g., hemorrhagic disease). The more serious form may kill between 90 and 100 percent of infected birds within two days.

Flu viruses typically infect a single species. Human instances of disease caused by bird flu are historically rare and have typically resulted in minor symptoms. This has changed somewhat with H5N1, the recent viral strain that has infected people. It hospitalized 18 people in Hong Kong in 1997. Of that group, six died. Since 2003, cases of human infection have been reported in 14 countries. The most cases have occurred in Indonesia, Vietnam, Egypt, China, Thailand and Turkey. According to the World Health Organization, the H5N1 virus has been linked to 351 cases of infection and 217 deaths between 2003 and the end of 2007. Each case was attributed to probable patient exposure to infected birds or possibly limited person-to-person transmission.

Like all viruses, the one that causes bird flu is a tiny particle that depends on infecting the cells of other organisms to reproduce. It is a member of the same group of viruses that causes “typical” flu each year in people.

When viewed through a microscope, influenza viruses look like a ball with a spiked surface. Inside the ball is the genetic material (RNA or ribonucleic acid) necessary for the virus to reproduce. The ball itself is made largely of a mix of proteins that form the virus membrane. Scientists classify the virus as influenza A, B or C, based on which proteins are in the membrane. The influenza A viruses are responsible for the majority of flu outbreaks and include bird flu.

The “spikes” seen on influenza A viruses are surface proteins embedded in the membrane of the ball. There are two types of surface proteins present, one of 16 varieties of hemagglutinin (H1 to H16) and one of nine varieties of neuraminidase (N1 to N9). Many combinations of proteins are possible. Those that include H1, H2 and H3 are typically involved in human disease. Viruses that cause bird flu that have also been transmitted to people have included H5, H7 and H9.

Viruses reproduce by invading cells and forcing them to create new viruses. The body’s immune system responds to viruses and other antigens 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.

Influenza viruses are able to change or mutate their structure over time to bypass the antibodies produced by the immune system. This allows influenza 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 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. For example, a bird flu virus and a human virus can both exist in a pig 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 influenza pandemic that caused millions of fatalities worldwide.