HUNDE, China — An hour south of Guangzhou, the Dongyuan animal market presents endless opportunities for an emerging germ. In hundreds of cramped stalls that stink of blood and guts, wholesale food vendors tend to veritable zoos that will grace Guangdong Province's tables: snakes, chickens, cats, turtles, badgers, frogs. And, in summer, sometimes rats, too.They are all stacked in cages one on top of another — which in turn serve as seats, card tables and dining quarters for the poor migrants who work there. On a recent morning, near stall 17, there were beheaded snakes, disemboweled frogs and feathers...