Guangdong, alias Guangzhou. It is not difficult to find pens in any village in this province of southern China. Dozens of pigs, goats, hundreds of chickens, and ducklings occupy each one. Generally, at least two herders live in a small room beneath its roof. They share the same air, smells, and maybe even the same viruses as their charges.

The worldwide medical community has now devoted its full attention to these villages. Last week, the World He