“DNA analysis has also shed light on the origin of the tapeworm, one of the 400 or so nonmicrobial parasites that regard the human body as home. The lifecycle of the tapeworm Taenia asiatica alternates between people and pigs, an animal that the religious authorities of both Judaism and Islam agree is unclean. It would be of interest to know just when these filthy animals infected people with their parasites. But the answer is not quite what had been expected.

Eric P. Hoberg, of the U.S.D.A.’s Agricultural Research Service in Beltsville, Md., concluded in 2001 that people contracted tapeworms millions of years ago in Africa, long before the emergence of agriculture and domestic animals. It was humans who infected pigs with tapeworms, not the other way around, Dr. Hoberg and colleagues reported. Indeed, people infected pigs not only with Taenia asiatica but also with a second species of tapeworm, Taenia solium, which humans seem to have acquired either by eating each other or by eating dogs.

If pigs had a religion, it is pretty easy to guess which species they would designate as unclean.”