Nearly a dozen Boston-area communities are now listed at high risk for West Nile virus after a fourth case of the mosquito-borne virus was discovered, the state Department of Public Health announced Monday.

The infected person is a woman in her 50s from Middlesex County who was never hospitalized for the illness, health officials said. The other three cases, which were reported on Friday, involve a Worcester County woman in her 70s, a woman in her 60s from Middlesex County, and a woman in her 50s from Suffolk County.

At least two of the four patients were exposed to the virus in the Greater Boston area, health officials said. As a result, state workers have raised the risk level from moderate to high for 11 communities — Arlington, Boston, Belmont, Brookline, Cambridge, Chelsea, Everett, Medford, Newton, Somerville, and Watertown.