How do the worst epidemics begin?

Viruses and bacteria must somehow jump from their host species to ours. Unfortunately, we enable infection when we kill and eat animals. This is true of the scariest and most deadly diseases to emerge in recent years: HIV (apes / bushmeat), avian bird flu (fowl), SARS coronavirus (exotic Chinese livestock).

Kristof writes about a MRSA epidemic in a small pig-farming community.

In some cultures, pigs are considered extremely dirty. This actually isn't so far from the truth, as pigs are natural reservoir hosts for many viruses and bacteria. The scary thing about pigs is that viruses and bacteria routinely recombine in their bodies .. in other words, a healthy pig carrying two viruses could transmit a new, extremely dangerous recombinant virus that is resistant to existing anti-retroviral drugs.