Reservoir rats

摘要

Between 1346 and 1353 the Black Death killed over a third of Europe's population. It took150 years for the continent to recover. The disease was so devastating that it changed the social order, as a scarcity of labour led to higher wages for the survivors, hastening the demise of feudalism. The plague was caused by Yersinia pes-tis, a bacterium that lives in fleas. In Europe, those fleas lived mostly on black rats (pictured). In Asia, where the disease came from, they lived on gerbils. It was thus a zoonotic illness: one usually carried by animals, but which infects people when given the chance. Since human beings have little evolutionary experience with such illnesses, and therefore little resistance to them, they can be particularly dangerous. Ebola fever is a zoonosis. So, as their names suggest, are the swine- and bird-flu strains that keep epidemiologists awake at night.