"Once the virus reached North America, it mutated, mixing with North American avian influenza strains to create the new viruses now being seen. The H parts, which are highly pathogenic in poultry, originated in Asia, and the N parts come from North American, low pathogenic, avian flu viruses, said Dr. Rubin Donis, an associate director for policy and preparedness in the Centers for Disease Control and Prevention's influenza division.
The U.S. Department of Agriculture is still seeing the original H5N8 virus, but it has also identified two strains of mixed-origin viruses, both of them highly pathogenic. One is the deadly H5N2 virus. The other is an entirely new H5N1 virus that has so far been found in only a handful of the recent cases."