A viruses found mainly in birds, but infections can occur in humans. Not all bird flu strains produce serious illness, but the current H5N1 strain is unusually pathogenic and has been shown to be transmissible to humans.Unlike most bird flu viruses, the H5N1 virus can be passed easily from birds to humans without first incubating in other animals such as pigs. Most cases have been traced directly to contact with sick poultry, and human-to-human transmission appears to have occurred in at least one instance. According to the World Health Organization, as of July 26, 2006 there have been 232 cases of flu worldwide, mostly in Southeast Asia. More than half the cases have been fatal. These statistics include only cases in which diagnosis has been confirmed by lab tests. However, the true incidence may be higher, and the true death rate lower, since cases of severe illness or death are most likely to be diagnosed and reported, while milder cases go unnoticed and unreported.

Infected birds shed influenza virus in their saliva, nasal secretions, and feces. Domesticated birds may become infected through direct contact with other birds or through contact with contaminated surfaces, water or feed. There are two main forms of avian flu that are distinguished by low and high extremes of virulence. The “low pathogenic” form can go undetected and usually causes only mild symptoms in birds, such as ruffled feathers and decreased egg laying. However, the highly pathogenic form spreads rapidly through poultry flocks, affects multiple internal organs and has a mortality rate of up to 90-100% within 48 hours. The virus can persist for more than a month in infected materials such as bird feces. Symptoms of avian influenza in humans have ranged from typical human influenza-like symptoms such as cough, fever, sore throat, and muscle aches to eye infections, pneumonia, severe respiratory distress, and other severe and life-threatening complications.