The Massachusetts Department of Public Health announced the fourth human case of West Nile virus on Monday.
Health officials said the victim is a woman in her 50s from Middlesex County who was never hospitalized for her illness.
Three other human cases were reported on Friday in addition to a horse. The horse had to be euthanized.
Days before the West Nile cases were reported, high numbers of mosquitoes testing positive for the virus prompted health officials to raise the risk level across the state from low to moderate. Friday's announcement then prompted health officials to raise the risk level from moderate to high for 11 communities in the Greater Boston area. Authorities said the communities impacted are Arlington, Boston, Belmont, Brookline, Cambridge, Chelsea, Everett, Medford, Newton, Somerville, and Watertown.
"Several individuals from the same area have developed West Nile virus,” said Public Health Commissioner Monica Bharel. "That means that there is an increased risk in this specific area and that additional people could become infected."
The human cases last week were identified as a woman in her 70s from Worcester County, a woman in her 60s from Middlesex County and woman in her 50s from Boston.
Bharel said health officials are particularly concerned about people over 50 and those who have immunodeficiency issues.
Officials say most people infected with West Nile virus won't present symptoms; however, when they do occur, they tend to manifest as fever and flu-like illness, with more serious illness presenting in rare cases.
Experts recommend people apply insect repellent when outside, be aware of peak mosquito hours and wear long sleeves, long pants and socks to keep mosquitoes away from skin.
"It is extremely important for people to take steps to avoid mosquito bites, including using repellents, wearing clothing to reduce exposed skin, dumping standing water, and moving indoors when you notice mosquitoes biting you," said DPH State Epidemiologist Dr. Catherine Brown.
In 2017, there were six human cases of West Nile virus infection identified in Massachusetts.