Massachusetts health officials on Aug. 10 confirmed the state's first human case of Eastern Equine Encephalitis virus since 2013.
Officials did not disclose the patient's identity, but said he is a man over age 60 who lives in Plymouth County.
The case has prompted the health department to increase the risk level for EEE to "critical" in nine communities.
"We are asking residents to take this risk very seriously," Massachusetts Public Health Commissioner Monica Bharel, MD, said in a press release. "We will continue to monitor this situation and the impacted communities."
EEE virus is spread by mosquitos and can cause a rare, sometimes fatal brain infection in humans. About 30 percent of people who contract EEE die, according to the CDC.