The scientists who solved the puzzle are from both Stanford and the Kaiser-Permanente Medical Center in Santa Clara, Calif. The authors said they did not find the entire worm, which has not been named, but identified it from fragments by using the genetic techniques.

The scientists do not know how often the new worm causes disease in humans and where the microbe exists in nature, Dr. David Relman, a co-author, said in an interview.

In recent years, scientists have identified a number of new microbes, including one called cyclospora, which is causing intestinal illness in at least 11 states. Health officials have tentatively linked many such cases to eating fresh fruits like raspberries and strawberries.

The man who died from the worm infection was a 44-year-old accountant from the San Francisco Bay area who had been infected with the AIDS virus for five years. He often went camping in California and had two dogs but had never traveled outside the United States.

In early 1994 he began suffering pain in his abdomen and back, weight loss, sweating at night and fever. After the man was admitted to the hospital in Santa Clara in March, his abdomen swelled as the worm invaded and destroyed parts of his intestine and liver. Doctors initially thought he had developed one of the many types of cancer that often complicate the course of someone with AIDS. Shortly before his deaths nine weeks later, doctors performed a surgical procedure to remove tissue from his abdomen.