ATHENS, Ga.- Dan Colley doesn't look like a Brazilian celebrity. The affable New York native doesn't even speak fluent Portuguese.

But the University of Georgia professor's study of a deadly parasite just won him the Brazilian Presidential Medal for Scientific Merit, the country's highest scholarly honor.

As head of the school's Center for Tropical and Emerging Diseases, Dr. Colley and his colleagues study some of the deadliest contagions on the planet. He delved into bacterial plagues and attacked diarrheal illness aboard cruise ships as the head of parasitic diseases for the Atlanta-based Centers for Disease Control and Prevention before sharpening his focus on his most puzzling scourge yet: schistosomiasis, a microscopic worm that plagues more than 200 million people across the globe.

The worms live in a certain type of snail that thrives only in freshwater lakes, usually in the tropics. Once the tiny parasite bores into an unfortunate host, it settles with a partner into a blood vein near the liver, where it mates for the rest of its life, as long as 40 years.

"Mate and eat. Stupid worms. That's all they do," Dr. Colley said with a laugh, gesturing at an inflated picture of mating parasites on his office door.

Some victims are completely resistant to infection. Others, though, suffer for decades with symptoms such as fatigue and stunted growth as their immune systems fight with too much vigor the droves of eggs the worms lay, walling them off with white blood cells that can scar veins and eventually cause severe internal bleeding. About 10 percent of victims - or 200,000 people - die from the parasite each year.

"Why? How does the immune system create a balance?" Dr. Colley asked. "If we knew why some got sick and others didn't, we'd learn some key things about the regulatory immune response - not just schistosomiasis, but Krohn's disease, arthritis, even allergies."

The disease, also known as snail fever, is in a class of parasite-borne ailments overlooked by some pharmaceutical companies because they mostly sicken the world's poorest people. To that end, Dr. Colley's helped assemble at UGA one of the nation's largest group of experts who specialize in the study of the parasites to lay the groundwork for treatments.

Dr. Colley's passion for these parasites got off to a slow start. He never took a class on deadly worms in school and hadn't even heard of schistosomiasis until a professor asked for volunteers to study in Brazil while Mr. Colley was a doctorate student at Tulane University.

Since that trip - the first of 80 to Brazil - he has tracked the parasite to St. Lucia, Egypt and Kenya to unearth the worm's secrets.

He and a research partner are now studying 200 workers in the west Kenyan city of Kisumu to determine why some are resistant to the disease while others are so vulnerable. The residents know the hazards of their local lake but still risk daily exposure to the parasite by standing in chest-deep water to shovel up sand into waiting boats.