A mosquito is seen at work in Franklin in 2008. (Photo: Debris Field via Flickr)

A Cambridge woman has been diagnosed with West Nile virus, one of five human cases of the mosquito-borne illness reported so far in Massachusetts, the city’s Public Health Department said Tuesday.

The woman, in her 60s, became ill July 30 with a fever and did not need to be hospitalized, the state Department of Public Health said. Another three women diagnosed with the virus suffered more serious symptoms of encephalitis and meningitis. Two of them, a Suffolk County resident in her 40s and a Worcester County woman in her 70s, were admitted to hospitals. A second Middlesex County woman in her 50s had meningitis but was not hospitalized. A Boston man in his 50s was reported with West Nile on Friday.

Most people infected with West Nile have no or few symptoms, but some suffer serious inflammation of the brain. Those “at greatest risk of serious illness” are people over 50 or who have impaired immune systems, the Cambridge Public Health Department says.

West Nile virus, first identified in Uganda, was initially detected in the United States in New York City in 1999 and was found in Massachusetts the next year. Birds provide a reservoir for the virus, and mosquitos that bite infected birds spread West Nile to humans. The virus has become established across the country.

In Massachusetts most people diagnosed with the virus become ill in August and September, when the virus becomes more widespread in mosquitoes. On Aug. 27, state health officials raised the risk level for the virus to high from moderate in Cambridge and 10 neighboring communities, city health officials said. That designation was based on multiple mosquito pools testing positive for West Nile.

On July 26, one sample in Cambridge tested positive; on Aug. 23, there were three positive samples. On Friday, the fifth.

Health officials advise residents to protect themselves against the virus by wearing mosquito repellent and pants and long-sleeved shirts outdoors. People should also consider staying indoors during times of high mosquito activity such as dawn and dusk, and keep screens in good repair. Residents can reduce the mosquito population by draining or eliminating containers that hold water, health officials say.

Information about the virus is on the Public Health Department website.

This story was updated Aug. 31, 2018, with news of a fifth human case of West Nile virus being identified in Massachusetts.