To date, there have been no cases of the H5N1 avian influenza that were spread from one human to another. All persons who have contracted this disease have either been infected through direct contact with birds, alive or dead, who were carrying the H5N1 virus or through contact with materials (e.g., tools, work surfaces) that were contaminated with the virus. The primary concern is that it will mutate to a form that is readily transmissible between humans. H5N1 is the most contagious and most severe of all recent avian influenza viruses, with a death rate of approximately 50% to date. Because the avian influenza viruses are only rarely contracted by humans, we do not have any in-built protection against them. Human populations gradually build tolerances to viruses they are regularly exposed to, which is why most forms of influenza do not produce high death rates. Barring the long-term, low-level exposure to such diseases, human populations usually experience unusually high death rates from new viruses. Even if a mutated form of H5N1 were to have only a 1% death rate, it would still mean that millions could die in North Amnerica alone.