Drake provides his perspective on "[t]hree theories that could explain why the North American H5N1 epidemic has not been more deadly" in humans.

1. "The molecular hypothesis posits that H5N1 viruses circulating in the Americas are intrinsically milder than those that circulated in Asia and Africa." In short, the NA H5N1 mostly lack mutations associated with virulence in mammals.
   
2. Epidemiological: Serological surveys in Michigan and Colorado dairy farmworkers and bovine veterinarians of the American Association of Bovine Practitioners (AABP) demonstrated that individuals had antibodies without knowing they were infected. The high mortality in mammals may be due to predation or scavenging on infected birds and the ingestion of large infectious doses.
3. Immunological: H1N1 exposure (i.e., "flu") may provide protection against H5N1.