The global spread of (highly pathogenic) H5N1 in birds is considered a significant pandemic threat.

The current H5N1 strain is a fast-mutating, highly pathogenic avian influenza virus (HPAI) found in multiple bird species.

It is both epizootic (an epidemic in non-humans) and panzootic (a disease affecting animals of many species especially over a wide area).

In January 2004 a major new outbreak of H5N1 surfaced in Vietnam and Thailand's poultry industry, and within weeks spread to ten countries and regions in Asia, including Indonesia, South Korea, Japan and China.

In October 2004 researchers discovered H5N1 is far more dangerous than previously believed because waterfowl were directly spreading the highly pathogenic strain of H5N1 to chickens, crows, pigeons, and other birds and that it was increasing its ability to infect mammals as well.