The global spread of (highly pathogenic) H5N1 in birds is considered a significant pandemic threat. While prior H5N1 strains have been known, they were significantly different from the current H5N1 strain on a genetic level, making the global spread of this new strain unprecedented. 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). Unless otherwise indicated, "H5N1" in this article refers to the recent highly pathogenic strain of H5N1.