Few images conjure up the horrors of primitive medical practice
more powerfully than that of the leech. This parasitic worm was used to suck blood from the veins of sick people in the belief that it could draw out the “evil vapors” responsible for their disease. Large numbers of leeches were employed. In 1837 alone, 96,000 leeches were applied to 50,557 patients at St. Bartholomew’s Hospital in London. Their use was so common in England that British-bred leeches became scarce, and foreign leeches had to be imported from India and Mexico.