While B viruses infect primarily humans, A viruses infect humans, pigs, horses, ferrets and many birds. Some researchers think that the sudden, pandemic-causing reassortment of surface proteins may occur in swine. Both human and bird viruses can infect a pig, co-mingle, and create a deadly subtype. If that new subtype makes its way into the human population, it can slaughter great numbers of people before it runs its course of infection throughout the population. An animal virus that jumps directly into the human body can also cause widespread devastation.⁸⁹

Historical records from two of this century's three major pandemics (in 1957 and 1968) suggest that those deadly strains formed in China, where vast numbers of people practice pig and duck farming in close quarters.⁹⁰

Unfortunately, the recognition of the vital animal-human link did not occur until decades later.