Sue Ryan, who lives in Plymouth, Mass., is taking no chances.

Plymouth is one of 10 communities in Massachusetts that health officials consider to be at high or critical risk from Eastern equine encephalitis, a rare but dangerous mosquito-borne disease that killed a 41-year-old man in New Hampshire this month. And Ms. Ryan is among those who have adjusted their routines accordingly.

“I’ve changed everything,” Ms. Ryan, 61, said on Wednesday morning as she visited a playground with her two grown daughters and three grandchildren. “I don’t go out on my patio after dark. I’ve stopped gardening. I will be obeying the rules until further notice.”

Plymouth’s parks, swimming ponds and playgrounds still thrummed with life on Wednesday. But Ms. Ryan was hardly the only resident who said she had begun taking precautions and heeding the advice of public health officials to stay indoors from dusk until dawn, when mosquitoes are most active.

The virus cannot be passed from person to person. Most people who are bitten by an infected mosquito never even become ill. But for the few who do, the virus can be very serious, leading to brain inflammation, neurological damage, coma or death. There is no treatment.

Between 2003 and 2023, Massachusetts reported a total of 41 human cases of the virus, more than any other state. Michigan and Florida, with 22 and 24 cases, ranked second and third, according to data from the Centers for Disease Control and Prevention provided by the Massachusetts Department of Public Health.

Southeastern Massachusetts, in particular, has seen its share of severe cases over the years; an outbreak there in 1938 killed 25 people, most of them children. More recently, in August 2006, a 9-year-old boy in Middleborough, Mass., died of the virus, falling into a coma one day after coming down with a headache and a fever.