Facts about Avian Influenza
Key Facts About Avian Influenza (Bird Flu) and Avian Influenza A (H5N1) Virus
This fact sheet provides general information about bird flu and information about one type of bird flu, called
avian influenza A (H5N1) that is infecting birds in Asia and has infected some humans. Also see the Frequently
Asked Questions (FAQs) on the World Health Organization (WHO) website.
What is avian influenza (bird flu)?
Bird flu is an infection caused by avian (bird) influenza (flu) viruses. These flu viruses occur naturally among birds.
Wild birds worldwide carry the viruses in their intestines, but usually do not get sick from them. However, bird flu
is very contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys,
very sick and kill them.
Do bird flu viruses infect humans?
Bird flu viruses do not usually infect humans, but several cases of human infection with bird flu viruses have
occurred since 1997.
How are bird flu viruses different from human flu viruses?
There are many different subtypes of type A influenza viruses. These subtypes differ because of certain proteins
on the surface of the influenza A virus (hemagglutinin [HA] and neuraminidase [NA] proteins). There are 16
different HA subtypes and 9 different NA subtypes of flu A viruses. Many different combinations of HA and NA
proteins are possible. Each combination is a different subtype. All known subtypes of flu A viruses can be found
in birds. However, when we talk about “bird flu” viruses, we are referring to influenza A subtypes chiefly found in
birds. They do not usually infect humans, even though we know they can. When we talk about “human flu
viruses” we are referring to those subtypes that occur widely in humans. There are only three known A subtypes
of human flu viruses (H1N1, H1N2, and H3N2); it is likely that some genetic parts of current human influenza A
viruses came from birds originally. Influenza A viruses are constantly changing, and they might adapt over time
to infect and spread among humans.
What are the symptoms of bird flu in humans?
Symptoms of bird flu in humans have ranged from typical flu-like symptoms (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 bird flu may depend on which virus caused the infection.
How does bird flu spread?
Infected birds shed flu virus in their saliva, nasal secretions, and feces. Susceptible birds become infected when
they have contact with contaminated excretions or surfaces that are contaminated with excretions. It is believed
that most cases of bird flu infection in humans have resulted from contact with infected poultry or contaminated
surfaces. The spread of avian influenza viruses from one ill person to another has been reported very rarely, and
transmission has not been observed to continue beyond one person.
How is bird flu in humans treated?
Studies done in laboratories suggest that the prescription medicines approved for human flu viruses should work
in preventing bird flu infection in humans. However, flu viruses can become resistant to these drugs, so these
medications may not always work. Additional studies are needed to prove the effectiveness of these medicines.
What is the risk to humans from bird flu?
The risk from bird flu is generally low to most people because the viruses occur mainly among birds and do not
usually infect humans. However, during an outbreak of bird flu among poultry (domesticated chicken, ducks,
turkeys), there is a possible risk to people who have contact with infected birds or surfaces that have been
contaminated with excretions from infected birds. The current outbreak of avian influenza A (H5N1) among
poultry in Asia and Europe (see below) is an example of a bird flu outbreak that has caused human infections and
deaths. In such situations, people should avoid contact with infected birds or contaminated surfaces, and should
be careful when handling and cooking poultry. For more information about avian influenza and food safety
issues, visit the World Health Organization website. In rare instances, limited human-to-human spread of H5N1
virus has occurred, and transmission has not been observed to continue beyond one person.
What is an avian influenza A (H5N1) virus?
Influenza A (H5N1) virus – also called “H5N1 virus” – is an influenza A virus subtype that occurs mainly in birds.
Like all bird flu viruses, H5N1 virus circulates among birds worldwide, is very contagious among birds, and can be
What is the H5N1 bird flu that has been reported in Asia and Europe?
Outbreaks of influenza H5N1 occurred among poultry in eight countries in Asia (Cambodia, China, Indonesia,
Japan, Laos , South Korea , Thailand , and Vietnam) during late 2003 and early 2004. At that time, more than 100
million birds in the affected countries either died from the disease or were killed in order to try to control the
outbreak. By March 2004, the outbreak was reported to be under control. Beginning in late June 2004, however,
new outbreaks of influenza H5N1 among poultry were reported by several countries in Asia (Cambodia, China [
Tibet ], Indonesia, Kazakhastan, Malaysia, Mongolia, Russia [ Siberia ], Thailand, and Vietnam). It is believed that
these outbreaks are ongoing. Most recently, influenza H5N1 has been reported among poultry in Turkey and
Romania. Human infections of influenza A (H5N1) have been reported in Cambodia, Indonesia, Thailand, and
What is the risk to humans from the H5N1 virus in Asia and Europe?
The H5N1 virus does not usually infect humans. In 1997. However, the first case of spread from a bird to a human
was seen during an outbreak of bird flu in poultry in Hong Kong, Special Administrative Region. The virus caused
severe respiratory illness in 18 people, 6 of whom died. Since that time, there have been other cases of H5N1
infection among humans. Recent human cases of H5N1 infection that have occurred in Cambodia, Thailand, and
Vietnam have coincided with large H5N1 outbreaks in poultry. The World Health Organization (WHO) also has
reported human cases in Indonesia. Most of these cases have occurred from contact with infected poultry or
contaminated surfaces; however, it is thought that a few cases of human-to-human spread of H5N1 have occurred.
So far, spread of H5N1 virus from person to person has been rare and has not continued beyond one person.
However, because all influenza viruses have the ability to change, scientists are concerned that the H5N1 virus
one day could be able to infect humans and spread easily from one person to another. Because these viruses do
not commonly infect humans, there is little or no immune protection against them in the human population. If
the H5N1 virus were able to infect people and spread easily from person to person, an influenza pandemic
(worldwide outbreak of disease) could begin. No one can predict when a pandemic might occur. However, experts
from around the world are watching the H5N1 situation in Asia very closely and are preparing for the possibility
that the virus may begin to spread more easily and widely from person to person.
How is infection with H5N1 virus in humans treated?
The H5N1 virus currently infecting birds in Asia that has caused human illness and death is resistant to amantadine
and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral medications, oseltamavir and zanamavir, would probably work to treat flu caused by the H5N1 virus, but additional studies still need to be done to prove their effectiveness.