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Hard frosts have eliminated the risk of the deadly Eastern equine encephalitis virus in areas of eight counties, and the unusually cold weather set to hit Massachusetts this week could kill more mosquitos.

A hard frost, a period of at least four consecutive hours of temperatures below 28 degrees, kills off the mosquitoes that carry the disease, according to the state’s Department of Public Health.

Berkshire, central Bristol, northern and central Essex, Franklin, Hampden, Hampshire, northern Middlesex, and Worcester counties have had hard frosts as of Monday, according to the DPH.

EEE is a rare but serious disease that has killed four people in Massachusetts this year. At its height, 35 communities were at critical risk of the disease. The department does not decrease risk levels at the end of the season, so the list of communities at critical risk will remain until the entire state sees a hard frost, Omar Cabrera, a spokesman for the DPH, said in an e-mail to the Globe.

In areas that have had a hard frost, the risk of EEE is eliminated except in places adjacent to an area where a hard frost has not occurred, Cabrera said.

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“At this time of the year it is also appropriate to remember that mosquito populations are generally low even in areas that have not had a hard frost,” he said.

The coldest air since last year’s winter is expected to arrive Thursday night, setting up a possibility of snow on Friday.

“Well below normal temperatures – more like early December than early November – look to spread across southern New England starting Friday. Add the winds, and it will feel even colder,” the National Weather Service said in a tweet Monday morning.

The cold temperatures are expected to hit Boston Thursday night as well, with a wind chill low of 23 degrees, according to the NWS. Friday will see a high of 38 degrees and a low of 26 degrees, and Saturday will be similar with a high of 37 degrees and a low of 28 degrees.

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Maria Lovato can be reached at maria.lovato@globe.com. Follow her on Twitter @maria_lovato99.