Afterward, he was an allergist at a pharmaceutical company, but the work bored him, and he returned to academia at Nottingham in 1981, investigating the ways parasitic worms suppressed rodents' immune systems.

"At that time," he said, "I realized the definitive work had to be carried out in humans. So I began to make inquiries about possible tropical venues for this research."

In the late 1980s, the Wellcome Trust issued a grant, and Pritchard and his Nottingham team set up camp on Karkar Island, Papua New Guinea.

"We didn't speak the language, and we were sparsely equipped," he recalled. "But we established a rapport with the people. We gave them worm tablets and would ask them politely, in pidgin English, to collect their fecal matter in buckets for us."

Hookworm infiltrates a victim's system when the larvae, hatched from eggs in infected people's excrement, penetrate the skin, often through the soles of the feet. From there, they enter the bloodstream, travel to the heart and lungs, and are swallowed when they reach the pharynx. They mature into adults once they reach the small intestine, where they can subsist for years by latching onto the intestinal wall and siphoning off blood. After sieving the fecal samples to extract hookworms eliminated when the worm treatment pill was given, the team reached an intriguing conclusion: Villagers with the highest levels of allergy-related antibodies in their blood had the smallest and least fertile parasites, indicating that these antibodies conferred a degree of protection against parasite infection.

And the hookworms seemed equipped to retaliate. After colonizing a digestive tract, the host often showed signs of a blunted immune response, leading Pritchard to suspect that the worms were reducing the potency of the body's defenses to make their environment more hospitable.

"Sitting in the jungle for long periods gives you time to think," he noted. "And this led to the idea that worm burdens of tolerable intensity could be beneficial under some circumstances."