In 2004, David Pritchard applied a dressing to his arm that was crawling with pin-size hookworm larvae, like maggots on the surface of meat. He left the wrap on for several days to make sure that the squirming freeloaders would infiltrate his system.

“The itch when they cross through your skin is indescribable,” he said. “My wife was a bit nervous about the whole thing.”

Dr. Pritchard, an immunologist-biologist at the University of Nottingham, is no masochist. His self-infection was in the interest of science.

While carrying out field work in Papua New Guinea in the late 1980s, he noticed that Papuans infected with the Necator americanus hookworm, a parasite that lives in the human gut, did not suffer much from an assortment of autoimmune-related illnesses, including hay fever and asthma. Over the years, Dr. Pritchard has developed a theory to explain the phenomenon.