The Massachusetts Department of Health says a Norfolk County woman who recently died is the state's first human case of Eastern equine encephalitis this year.
The health department identified the victim only as a woman in her 80s. She was hospitalized in mid-August and died a few days later.
Health officials are trying to determine where she was most likely exposed to infected mosquitoes.
There were seven cases of EEE in Massachusetts last year, and at least two deaths.
People are urged to take precautions against mosquitoes.
The health department says symptoms of EEE include fever, stiff neck and headache, and usually appear three to 10 days after a mosquito bite. The most serious complication is inflammation and swelling of the brain.