A man in his 80s who was exposed in Worcester County is the state’s first human case of Eastern equine encephalitis since 2020, and risk levels for the mosquito-borne illness have been elevated in that region, Massachusetts health officials announced Friday.

Generally spread to humans through the bite of an infected mosquito, the disease is rare, yet serious and potentially fatal to people of all ages, officials said.

The risk level has been increased to “critical” in Douglas, Oxford, Sutton, and Webster, according to the Massachusetts Department of Public Health.

A “high” risk level has been set in Dudley, Northbridge, and Uxbridge, officials said in a statement.

Meanwhile, nine neighboring communities are now considered at “moderate” risk: Auburn, Charlton, Grafton, Leicester, Mendon, Millbury, Millville, Southbridge, and Upton.

“EEE is a rare but serious disease and a public health concern,” Public Health Commissioner Robbie Goldstein said in the statement. “We want to remind residents of the need to protect themselves from mosquito bites, especially in areas of the state where we are seeing EEE activity.”

In 2019, 12 people were infected with EEE in Massachusetts and six died. In 2020, there were five human cases in the state and one death, the statement said. There were no human cases in the state in 2021, 2022, or 2023.

The season’s first EEE-positive mosquitoes in Massachusetts were announced on July 3rd from Carver, the statement said.

Earlier this month, the state said the state’s first animal case of EEE this year had been confirmed in a horse exposed in Plymouth County.

“The risk from EEE is high in parts of Plymouth County and critical in parts of Worcester County,” State Epidemiologist Dr. Catherine M. Brown, said in the statement. “In addition to recommending that people use mosquito repellent with an EPA-registered active ingredient and clothing to reduce exposed skin, we also recommend that evening outdoor events be rescheduled to avoid the hours between dusk and dawn. The mosquitoes most likely to spread EEE are most active during the dusk to dawn hours.”

Infected mosquitoes also have been identified in Abington, Halifax, Kingston, Middleborough, Plymouth, Wareham, and Whitman in Plymouth County, Taunton and Westport in Bristol County, Barnstable in Barnstable County, Dedham in Norfolk County, Sudbury in Middlesex County, and Amesbury and Haverhill in Essex County.

More information, including all EEE positive results, can be found on the Arbovirus Surveillance Information web page at Mosquito-borne Diseases | Mass.gov, which is updated daily, or by calling the DPH Division of Epidemiology at 617-983-6800.

Tonya Alanez can be reached at tonya.alanez@globe.com. Follow her @talanez.