Avian influenza virus usually refers to influenza A viruses found mainly in birds, but infections can occur in humans. The risk is generally low to most people, because the viruses do not usually infect humans. However, confirmed cases of human infection have been reported since 1997, and the current outbreak of bird flu worldwide increases the risk that the virus will mutate into a form which can be spread person to person.

All flu viruses are thought to have originated in birds. But scientists also have long believed that in order to cause human epidemics, the viruses first had to spread from birds to pigs, where genetic changes occur which allow the viruses to spread more easily in mammals. Flu strains that are more birdlike are more dangerous to humans because their immune systems have not been exposed to them before.

It is thought that the deadly 1918 flu epidemic, which killed 20 to 40 million people, derived from bird flu. Known as "Spanish Flu" or "La Grippe," the influenza pandemic of 1918-1919 was a global disaster, and it has been cited as the most devastating epidemic in recorded world history. More people died from influenza in a single year than in four years of the Black Death or Bubonic Plague from 1347 to 1351.

Obviously, it is of vital importance that bird flu be contained, and that any infected birds are culled. It would only require a handful of mutations in order for H5N1 avian flu to become a pandemic virus like the one which spread around the world in 1918. Once human-to-human transmission evolves, the disease could spread around the globe in days through modern jet travel by infected persons.

The current bird flu, a strain known as H5N1, clearly can be transmitted directly from poultry to people. Most cases have been traced directly to contact with sick birds, although human-to-human transmission appears to have occurred in at least one instance. H5N1 first appeared in Hong Kong in 1997, and as of July 26, 2006 there have been 232 cases of flu worldwide, mostly in Southeast Asia, according to the World Health Organization. Mortality has been greater than fifty percent. These figures include only patients whose diagnosis has been confirmed by lab tests. Cases of severe illness or death are most likely to be diagnosed and reported, while milder cases go undiagnosed and unreported; therefor it is difficult to know the true mortality rate from H5N1.

Due to the fact that most cases are occurring in third-world countries where health services and reporting are poor, the actual number is likely much higher than the official count. Masato Tashiro, head of virology at the Tokyo National Institute of Infectious Disease, gave a presentation at a virology conference in Marburg, Germany, on November 19, 2005 about unofficial bird flu tallies from China. He listed "several dozen" human outbreaks, nearly 300 deaths, 3000 people quarantined, and seven human-to-human transmissions. The meeting was reported in the German newspaper Frankfurter Allgemeine Zeitung.