An adult in New Hampshire has died from infection with Eastern equine encephalomyelitis, a rare but serious illness caused by a mosquito-borne virus, according to a statement from the New Hampshire Department of Health and Human Services (DHHS).

In the United States, three more cases of the disease, which can cause encephalitis in extreme situations, have been reported this year: one in Massachusetts, one in Vermont and one in Wisconsin, according to the Centers for Disease Control and Prevention (CDC).

According to the CDC, an average of 11 human cases of Eastern equine encephalomyelitis are reported each year in the United States. However, the disease is often fatal—about 30 percent of those affected die, and many survivors continue to have neurological problems. There are no vaccines or drugs to treat the disease.

Two counties in Massachusetts began spraying insecticides from the air and from trucks on Tuesday because of the risk of human disease. The city of Plymouth is now closing its public parks and fields from dusk to dawn because the city is at high risk, according to the New York Times“Amanda Holpuch.

Health authorities recommend that people take steps to avoid infection, including using repellents and wearing well-covering clothing when outdoors, and avoiding outdoor activities between dusk and dawn.

Most cases of Eastern equine encephalomyelitis in the United States occur in the eastern states and along the Gulf Coast. The virus is transmitted between mosquitoes and birds, which are typically found in freshwater and deciduous swamps. People can contract the virus through a mosquito bite, but infected people do not spread the virus to other people.

Most people infected with the virus do not develop symptoms—only about 4 to 5 percent develop Eastern equine encephalomyelitis, according to the CDC. People under 15 and over 50 are at greatest risk of becoming seriously ill. Since 2003, there have only been five years in which more than 10 cases were reported in the country. The year with the most cases during that period was 2019, with 38 cases.

People who develop symptoms usually do so between four and ten days after being bitten by an infected mosquito. Many infections cause fever, chills, body aches and joint pain, but in rarer cases infected people can develop neurological conditions such as inflammation of the brain, known as encephalitis, or inflammation of the membranes around the brain and spinal cord, known as meningitis. Symptoms of the more severe disease can include fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness and coma.

“Of the 4 to 5 percent who become infected and develop the disease, only about a third develop the most severe and horrific form of the disease, which is encephalitis,” Stephen Rich, a microbiologist at the University of Massachusetts Amherst, told the New York Times.

“If it causes an infection, it’s very, very serious. Even though it’s a very rare infection, we don’t have a treatment for it,” Richard Ellison, an infectious disease specialist at the University of Massachusetts Memorial Medical Center, tells Patrick Whittle of the Associated Press. “If someone gets it, it’s just — all we can do is supportive care, and it can kill people.”

The person who died in New Hampshire was hospitalized for severe central nervous system disease, according to the DHHS statement. No infection has been reported in the state since 2014, when there were three cases and two deaths. The case in Massachusetts is the state’s first since 2020, according to the Massachusetts Department of Public Health.

According to the New Hampshire DHHS, Eastern equine encephalomyelitis virus (EEEV) has been detected this year in seven mosquito batches in New Hampshire, 69 mosquito samples in Massachusetts and 47 mosquito samples in Vermont.

“We believe there is an increased risk of EEEV infection in New England this year because positive mosquito samples have been identified,” New Hampshire state epidemiologist Benjamin Chan said in the statement. “The risk will persist into the fall until a hard freeze occurs to kill off mosquitoes. Everyone should take steps to avoid mosquito bites when they are outdoors.”