Type A influenza viruses can infect several animal species, including birds, pigs, horses, seals and whales. Influenza viruses that infect birds are called "avian influenza viruses." Birds are an especially important species because all known subtypes of influenza A viruses circulate among wild birds, which are considered the natural reservoir for influenza A viruses. Avian influenza viruses do not usually directly infect humans or circulate among humans.

Influenza A viruses can be divided into subtypes on the basis of their surface proteins - hemagglutinin (HA) and neuraminidase (NA). There are 15 known H subtypes. While all subtypes can be found in birds, only 3 subtypes of HA (H1, H2 and H3) and two subtypes of NA (N1 and N2) are known to have circulated widely in humans during the last century.

Avian influenza usually does not make wild birds sick, but can make domesticated birds very sick and kill them. Avian influenza A viruses do not usually infect humans; however, several instances of human infections and outbreaks have been reported since 1997. When such infections occur, public health authorities monitor the situation closely because of concerns about the potential for more widespread infection in the human population.

Current Situation

Since December 2003, WHO has received reports of confirmed human cases and widespread poultry outbreaks of avian influenza A (H5N1) in Asia. Although the human cases are thought to have resulted from direct exposure to infected live poultry or their contaminated environment, limited human-to-human transmission may be possible. The exposure of humans to ongoing poultry outbreaks is a grave concern because it enhances the potential for avian influenza A (H5N1) viruses to undergo genetic changes or recombine with human influenza viruses and result in a new influenza A virus that is easily transmitted human-to-human, thus triggering an influenza pandemic. During the last century, 3 influenza pandemics resulted in millions of deaths worldwide.

In the United States, several states have reported avian influenza among poultry. However, the strains identified in the US are not the same subtype that is circulating in Asia. State and national authorities are monitoring the situation closely to prevent transmission to humans and to ensure widespread outbreaks do not occur.

For more information on avian influenza, please go to http://www.cdc.gov/flu/avian.

Division of Public Health Resources

Health Alert Network
Announcements released by local, state or federal public health authorities, meant to inform the health and medical services of likely or imminent dangers to the health of the community.

General Public

• Influenza Surveillance
• Influenza Information

Healthcare Providers

• Think Avian Flu! Poster 
• Influenza A (H5N1) and SARS recommendations: February 20, 2004 

CDC Resources

• Influenza
• Avian Influenza

Other Resources

• World Health Organization (WHO)
• World Organization for Animal Health / Office International des Epizooties (OIE)