Primary Navigation for the CDC Website
CDC en Español


Key Facts About Avian Influenza (Bird Flu) and Highly Pathogenic Avian Influenza A (H5N1) Virus

This website is archived and is no longer being maintained or updated. For updated information on avian influenza, see the CDC Avian Influenza website and the World Health Organization (WHO) Avian Influenza Situation Update page.

This fact sheet provides general information about avian influenza (bird flu) and information about one type of bird flu, called highly pathogenic avian influenza A (H5N1), also called "HPAI H5N1," that has caused infections in birds and in humans. Also see Questions and Answers on the CDC website.

Avian Influenza (Bird Flu)

Avian influenza in birds

Avian influenza is an infection caused by avian (bird) influenza (flu) A viruses. These influenza A viruses occur naturally among birds. Wild birds worldwide get flu A infections in their intestines, but usually do not get sick from flu infections. However, avian influenza is very contagious among birds and some of these viruses can make certain domesticated bird species, including chickens, ducks, and turkeys, very sick and kill them.

Infected birds can shed influenza virus in their saliva, nasal secretions, and feces. Susceptible birds become infected when they have contact with contaminated secretions or excretions or with surfaces that are contaminated with secretions or excretions from infected birds. Domesticated birds may become infected with avian influenza virus through direct contact with infected waterfowl or other infected poultry, or through contact with surfaces (such as dirt or cages) or materials (such as water or feed) that have been contaminated with the virus.

Infection with avian influenza viruses in domestic poultry causes two main forms of disease that are distinguished by low and high extremes of virulence. The "low pathogenic" form may go undetected and usually causes only mild symptoms (such as ruffled feathers and a drop in egg production). However, the highly pathogenic form spreads more rapidly through flocks of poultry. This form may cause disease that affects multiple internal organs and has a mortality rate that can reach 90-100% often within 48 hours.

Human infection with avian influenza viruses

There are many different subtypes of type A influenza viruses. These subtypes differ based on differences in two main proteins on the surface of the influenza A virus (hemagglutinin [HA] and neuraminidase [NA] proteins). There are 16 known HA subtypes and 9 known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible. Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds.

Usually, “avian influenza virus” refers to influenza A viruses found chiefly in birds, but infections with these viruses can occur in humans. The risk from avian influenza is generally low to most people, because the viruses do not usually infect humans. However, confirmed cases of human infection from several subtypes of avian influenza infection have been reported since 1997. Most cases of avian influenza infection in humans have resulted from contact with infected poultry (e.g., domesticated chicken, ducks, and turkeys) or surfaces contaminated with secretion/excretions from infected birds. The spread of avian influenza viruses from one ill person to another person has been reported very rarely, and has been limited, inefficient and unsustained.

"Human influenza A viruses" usually refers to those influenza A subtypes that have spread widely among humans. Currently, H3N2 and H1N1 influenza A subtypes are circulating among humans and H2N2 influenza A circulated from about 1957-1968.

Some genetic parts of current human influenza A viruses had their origin in bird flu viruses originally. Influenza A viruses are constantly changing, and they might adapt over time to infect and spread among humans.

During an outbreak of avian influenza among poultry, there is a possible risk of infection for people who have contact with infected birds or surfaces that have been contaminated with secretions or excretions from infected birds.

Symptoms of avian influenza in humans have ranged from typical human influenza-like symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections, pneumonia, severe respiratory diseases (such as acute respiratory distress), and other severe and life-threatening complications. The symptoms of avian influenza may depend on which virus caused the infection.

Studies done in laboratories suggest that some of the antiviral drugs approved in the United States for human influenza viruses should work in treating avian influenza infection in humans. However, influenza viruses can become resistant to these drugs, so these medications may not always work. Additional studies are needed to demonstrate the effectiveness of these medicines. When avian influenza A viruses are identified to cause illness in humans, the viruses should be tested for susceptibility to influenza antiviral medications.

Highly pathogenic Avian Influenza A (H5N1)

Highly pathogenic Influenza A (H5N1) virus – also called "HPAI H5N1 virus" – is an influenza A virus that occurs mainly in birds, is highly contagious among birds, and can be deadly to them, especially domestic poultry. HPAI H5N1 virus does not usually infect people, but infections with these viruses have occurred in humans. Most of these cases have resulted from people having direct or close contact with H5N1-infected poultry or H5N1-contaminated surfaces.

Human health risks from HPAI H5N1

Of the few avian influenza viruses that have crossed the species barrier to infect humans, HPAI H5N1 has caused the largest number of detected cases of severe disease and death in humans. However, it is possible that those cases in the most severely ill people are more likely to be diagnosed and reported, while milder cases are less likely to be detected and reported. For the most current information about avian influenza and cumulative case numbers, see the World Health Organization (WHO) avian influenza website.

Of the human cases associated with the ongoing HPAI H5N1 outbreaks in poultry and wild birds in Asia and parts of Europe, the Near East and Africa, about 60% of those people reported infected with the virus have died. Most cases have occurred in previously healthy children and young adults and have resulted from direct or close contact with H5N1-infected poultry or H5N1-contaminated surfaces. In general, HPAI H5N1 remains a very rare disease in people. The HPAI H5N1 virus does not infect humans easily, and if a person is infected, it is very difficult for the virus to spread to another person.

While there has been some human-to-human spread of HPAI H5N1, it has been limited, inefficient and unsustained. For example, in 2004 in Thailand, probable human-to-human spread in a family resulting from prolonged and very close contact between an ill child and her mother was reported. In June 2006, WHO reported evidence of human-to-human spread in Indonesia. In this situation, eight people in one family were infected. The first family member is thought to have become ill through contact with infected poultry. This person then infected six family members. One of those six people (a child) then infected another family member (his father). No further spread outside of the exposed family was identified.

Nonetheless, because all influenza viruses have the ability to change and because the HPAI H5N1 known ability to cause human infections, scientists remain concerned that HPAI H5N1 viruses have the potential to possibly change into a form of the virus that is able to spread easily from person to person. Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If HPAI H5N1 virus were to gain the capacity to spread easily from person to person, an influenza pandemic (worldwide outbreak of disease) could begin. For more information about influenza pandemics, see

Experts from around the world continue to monitor for potential changes in the HPAI H5N1 virus and changes in patterns of human infection and work with the many stakeholders to prepare for the possibility that the virus may begin to spread more easily and widely from person to person. For more information about influenza pandemics, see

Treatment and vaccination for HPAI H5N1 virus in humans

The HPAI H5N1 virus that has caused human illness and death in Asia is resistant to amantadine and rimantadine, two antiviral medications licensed by the US Food and Drug Administration (FDA) for influenza A. However, these drugs are not recommended for influenza treatment at this time because of resistance among both HPAI H5N1 and resistance against circulating human influenza A H1N1 and H3N2 viruses. . Two other antiviral medications, oseltamivir and zanamivir, can be used for the treatment of HPAI H5N1 although WHO guidance includes a preference for oseltamivir. For more information on the treatment of HPAI H5N1, visit:

Page last modified November 21, 2010

Navigation for the CDC Avian Flu Website

• Influenza (Flu)

Additional Navigation for the CDC Website

“Safer Healthier People”Centers for Disease Control and Prevention, 1600 Clifton Rd, Atlanta, GA 30333, U.S.A.
800–CDC–INFO (800-232-4636), 888–232–6348 (TTY),