Bird flu is an infection in humans that is caused by strains of influenza virus that usually infect only wild birds and domestic poultry.
The bird flu virus rarely spreads from animals to people.
Nearly all people who have been infected with bird flu have had close contact with an infected bird (it rarely spreads from one person to another).
People may have extreme difficulty breathing and flu-like symptoms (fever, cough, and headache).
To diagnose bird flu, doctors test a sample of secretions taken from the person's nose or throat.
Bird flu is treated with antiviral medications.
(See also Overview of Viral Infections and Influenza.)
Bird flu is caused by several strains of influenza A virus that originally came from wild birds. These strains can spread to other birds, such as farm birds (chickens and turkeys), pet, and zoo birds, and to other animals, such as seals, and cause the infection. The infection can then spread from animals to people if the genetic material of the virus changes (mutates), enabling the virus to attach to cells in the human respiratory tract. Infections that can be transmitted between people and animals are called zoonotic diseases, or zoonoses (see Introduction to Diseases Spread Between Animals and People (Zoonoses)). (See also Avian Influenza (Bird Flu).) Nearly all people who have been infected with bird flu have had close contact with an infected bird. Bird flu only rarely spreads from person to person.
Strains of influenza A virus are named based on the properties of the 2 proteins that are present on the surface of the virus. The proteins are hemagglutinin (H) and neuraminidase (NA). To date, 18 subtypes of H and 11 subtypes of NA have been identified in influenza viruses. Subtypes are numbered, so, for example, a virus with subtype 1 of hemagglutinin and subtype 1 of neuraminidase is named H1N1. The H1N1 strain caused the swine flu pandemic in 2009–2010. (A pandemic is a sudden, unexpected outbreak of a disease that spreads rapidly and globally.) The H5N1 and H7N9 strains cause most cases of bird flu in people.
This illustration shows a generic influenza A virus. Covering the surface of the virus are the proteins hemagglutinin (H; light blue) and neuraminidase (NA; red), which determine the strain of virus.
CDC/Illustrator: Dan Higgins
Human infection with the avian influenza strain H5N1 (see influenza types and strains) first occurred in Hong Kong in 1997 and spread to other nations in 2003 and 2004. In 2024, H5N1 was reported among dairy cows in several states in the United States. The outbreak that occurred caused 71 infections and 2 deaths in humans between 2024 and 2025. There has been no known person-to-person spread.
In 2013, an outbreak of avian flu strain H7N9 that infected humans occurred in southeastern China. The infection occurred mainly in people who were directly exposed to infected birds at live (wet) poultry markets. A sixth wave of H7N9 infection in humans in China peaked in 2016–2017 with nearly 800 cases, and only sporadic cases have been reported since that time. Worldwide, 1,568 human infections and 616 deaths have been reported to the World Health Organization since 2013.
Other strains of the avian flu virus have also caused sporadic outbreaks of infection in people in other parts of the world, such as Canada and the Netherlands.
Symptoms of Bird Flu
The H5N1 and H7N9 strains cause similar symptoms.
People may have flu-like symptoms (such as fever, cough, headache, sore throat, and muscle aches). Some people have conjunctivitis (inflammation in the conjunctiva, which is part of the eye). Some have difficulty breathing and may develop complications such as pneumonia.
Diagnosis of Bird Flu
Testing of secretions taken from the nose or throat
People who have flu-like symptoms (fever, cough, and headache) and who have had contact with birds in an area where birds are known to be infected with bird flu, contact with dairy cows, or contact with a person infected with bird flu should see a doctor.
The doctor swabs the person's nose or throat to obtain samples of secretions that can be tested in a laboratory.
Treatment of Bird Flu
Antiviral medications
Infected people are given oseltamivir or zanamivir or sometimes baloxavir (antiviral medications that are used to treat influenza).
Prevention of Bird Flu
People should avoid contact with sick or dead animals, especially wild and domesticated birds, poultry, and cattle. Raw or undercooked food products, such as unpasteurized milk and dairy products, should also be avoided. The commercial milk supply in the United States is regularly tested, and no live, infectious bird flu viruses have been found in pasteurized milk. Dairies are required to follow food safety practices, including processing milk only from healthy animals for human consumption, and pasteurization is required for milk to be sold in stores. (See also Centers for Disease Control and Prevention: Preventing Bird Flu Infections.)
Poultry in China are given a vaccine to prevent H5 and H7 bird flu viruses. This vaccination program helps prevent the bird flu virus from spreading from wild birds to domestic birds. Domestic birds are more likely to come in contact with people and spread the virus to them.
Spread is further prevented by intentionally removing and eliminating (culling) infected flocks of domestic birds.
In the United States, a vaccine for people against H5N1 bird flu is available for distribution if deemed necessary by public health authorities. The standard vaccine for influenza does not prevent bird flu.
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