infection caused by avian influenza (bird flu) viruses; also called bird flu. These 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. Bird flu viruses do not usually infect humans, but several cases of human infection with bird flu viruses have occurred since 1997. 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. An outbreak of avian influenza A (strain H5N1) among poultry in Asia (October 2005) is an example of a bird flu outbreak that 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. 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. Laboratory testing to confirm human infection with H5N1 avian influenza is technically difficult; some tests produce inconclusive or unreliable results. Currently, World Health Organization (WHO) reference laboratories must be used for diagnostic confirmation. The H5N1 virus currently infecting birds in Asia that has caused human illness and death is resistant to amantadine (Symmetrel) and rimantadine (Flumadine), two antiviral medications commonly used for influenza. Two other antiviral medications, oseltamavir (Tamiflu) and zanamavir (Relenza), can be used for treatment and prophylaxis. There currently is no vaccine to protect humans against the H5N1 virus that is being seen in Asia. However, vaccine development efforts are under way. Research studies to test a vaccine to protect humans against H5N1 virus began in April 2005.