Willy Burgdorfer, a medical entomologist who in 1982 identified the cause of what had been a mysterious affliction, Lyme disease, died on Monday at a hospital in Hamilton, Mont. He was 89.

The cause was complications of Parkinson’s disease, said Tom Schwan, a colleague of Mr. Burgdorfer’s for many years at the Rocky Mountain Laboratories of the National Institute of Allergy and Infectious Diseases in Hamilton.

Lyme disease took its name from the Connecticut area where it first drew attention in the 1970s, including the towns of Lyme, Old Lyme and East Haddam. Scores of people in the area, particularly children, had developed rashes, fevers, swollen joints and sometimes more serious symptoms. Early on, it was called Lyme arthritis. It did not take long for scientists to speculate that there was a common link: blacklegged ticks, also called deer ticks.

Many of the children lived and played in wooded areas and developed rashes after being bitten by deer ticks. Cases were more prevalent east of the Connecticut River, where there were more deer. Many laboratory hours were devoted to determining if the deer ticks were spreading a virus. But no virus was detected. There had to be another explanation.