A man in his 70s has become the sixth Massachusetts resident infected with West Nile virus and was likely exposed in Middlesex County, the state Department of Public Health said Wednesday.
The county remains at moderate risk for the mosquito-borne virus, the statement said.
Despite colder temperatures, some risk for the virus will continue until areas have a “hard frost,” or when temperatures fall below 28 degrees, the department said.
The risk of West Nile also remains moderate in parts of Greater Boston, Norfolk, and Suffolk counties, the statement said. Risk also remains moderate in Berkshire, Bristol, Hampden, Hampshire, Plymouth, and Worcester counties.
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The department advises residents to protect themselves by applying insect repellent, avoiding peak mosquito hours (dusk and dawn), and repairing window screens to keep mosquitoes outside.
The virus is usually transmitted to humans by a bite from an infected mosquito, the department said. People age 50 and older are at higher risk for severe disease.
Most people infected with West Nile show no symptoms, the statement said. If symptoms do show, they typically include fever and flu-like illness.
Last year, there were eight human cases of the virus in Massachusetts, according to the department.
Maeve Lawler can be reached at maeve.lawler@globe.com. Follow her @maeve_lawler.