Avian influenza or “bird flu” first made headlines in 1997, when the H5N1 virus caused an outbreak among poultry in Hong Kong. Within 3 days, Hong Kong's entire poultry population of 1.5 million birds was culled, and the outbreak was temporarily contained. The virus reappeared in Asia in December 2003. By 2004, several Southeast Asian countries were reporting outbreaks in poultry, and human cases were reported from Thailand and Vietnam.

Despite massive culling operations, the disease continued to spread and by July 2005 was reported in Siberia, Kazakhstan, Mongolia, and Croatia. The number of human cases also continued to rise. In addition, cases of fatal avian influenza were reported in civets, zoo leopards and tigers, and domestic cats—species that were not previously thought to be susceptible. In 2006, disease spread to several countries in Western Europe and appeared in Africa for the first time. The human case count has now exceeded 200 (as of September 2006), with mortality of approximately 50%.

The virus continues to mutate and evolve, and two distinct strains or clades have emerged—the original index strain that initiated the outbreak (Clade 1) and is circulating in Cambodia, Laos, Malaysia, Thailand, and Vietnam; and another strain (Clade 2) called the Indonesia strain that is circulating in Indonesia, China, Japan, and South Korea.