In Pandemic, Den Beste brings up an interesting point about influenza:
One of the dirty little secrets in infectious disease research is that most new strains of flu come from China. It doesn’t get talked about much because there really isn’t much that can be done about it.
It turns out that they emerge as the result of genetic crosses between human flu and avian flu, which is mostly a disease of geese and ducks. But humans don’t easily get avian flu, and birds don’t easily get human influenza. However, pigs can get both quite easily, and if a given pig is infected with both simultaneously, then it’s possible for one of its cells to have both kinds of viruses inside at the same time, merrily hijacking the cell’s mechanisms to make more viruses. In that case, there’s a chance of genetic mixing.
So it turns out that this is most likely to happen in places where humans, geese or ducks, and swine all live in close proximity in relatively primitive and unclean conditions, and that turns out to mean China’s collectivized farms. Similar conditions exist elsewhere but they’re much less common. More modern industrialized livestock farming, such as is practiced in the US, isn’t susceptible to this risk.