She had the fourth confirmed case of EEE in Massachusetts since August.

A Massachusetts woman has been identified as having eastern equine encephalitis (EEE)—the fourth confirmed case of the rare but dangerous infection in the state this year.

The Massachusetts Department of Public Health (DPH) announced on Sunday via press release that an unidentified woman over the age of 50 from southern Bristol County was infected with the disease. While the DPH didn't identify the woman or give an update on her condition, Robert Sylvia, Jr., told NBC10 Boston that the unnamed woman was his wife, Laurie Sylvia, who died Sunday from the disease.

According to Sylvia Jr., his wife began feeling sick Monday of last week, and died in the hospital on Saturday. An autopsy is underway, but according to NBC10, Syliva Jr. is "convinced his wife died from the virus spread by mosquitoes."

The three other confirmed cases of EEE in Massachusetts were diagnosed in a Rochester man older than 60, a man in Grafton between 19 and 30, and a man over 60 years old in northern Franklin County, according to multiple DPH press releases. Before this, Massachusetts hadn't seen an EEE case in humans since 2013.

What is eastern equine encephalitis, exactly?

Eastern equine encephalitis—or, again, EEE—is a rare disease caused by a mosquito-borne virus (EEEV). EEEV is one of a group of mosquito-borne viruses that can cause inflammation of the brain (aka, encephalitis), according to the Centers for Disease Control and Prevention (CDC).

Because it bears repeating: EEE is rare, and only about 4-5% of human EEEV infections result in EEE. Per the CDC, about seven human cases of EEE are reported each year—and about 30 percent of people who contract EEE die, while many who survive have lasting neurological problems.

Also important to note: The EEE virus can cause two different types of illness, per the CDC: systemic or encephalitic (which, again, causes inflammation and swelling in the brain). In some cases, the systemic form precedes the encephalitic form of the virus.

What are the symptoms of EEE?

Once someone is bitten by an infected mosquito, the EEE virus can incubate for four to 10 days before a person starts to show symptoms—though, again, some people never develop EEE or even show symptoms, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the John's Hopkins Center for Health Security.

In those who do show symptoms, though, the systemic form typically causes flu-like symptoms including chills, fever, arthralgia (joint pain), and muscle pain, the CDC says. The sickness usually lasts for one to two weeks, and, if they don't develop any central nervous system issues, patients make a full recovery.

The encephalitic form of EEE is a bit more complicated: In children, encephalitis caused by EEEV typically comes on abruptly, per the CDC, but in older kids and adults, encephalitis manifests after a few days of systemic illness. In that case, symptoms progress and can include fever, headache, irritability, restlessness, drowsiness, vomiting, diarrhea, a bluish tint to their skin, convulsions, and coma, per the CDC.

How is EEE treated—and how can you prevent it?

Unfortunately, there is no specific antiviral medication or treatment for EEE. “You treat the symptoms,” Dr. Adalja says. “Often these patients are very sick and may be in the intensive care unit.”

Luckily, you can avoid EEE as much as possible by preventing mosquito bites in general. The best ways to do that, according to the CDC, include using insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing, as well as covering up exposed skin by wearing long sleeves and pants as much as possible.

Mosquitos also love standing water, so the CDC also recommends getting rid of any water build-up in flower pots, buckets, barrels, and even children's kiddie pools. Another smart strategy: Stay away from freshwater hardwood swamps—mainly in the Atlantic and Gulf Coast states and the Great Lakes region, where EEEV transmission is most common, per the CDC.

So all of this is pretty scary, but how worried do you really have to be about EEE?

The truth: While you should definitely be aware of EEE, you shouldn’t panic. “It is still a very rare infection,” says Richard Watkins, M.D., an infectious disease physician in Akron, Ohio, and an associate professor of internal medicine at Northeast Ohio Medical University. Dr. Adalja agrees. “It’s not the most common mosquito-borne illness,” he says. “You’re more likely to get West Nile, but the consequences of EEE make people pay more attention to it.”

EEE is most common in swampy marsh areas like the Gulf states and Massachusetts, Dr. Adalja says, and people who live in those areas should be aware of the disease. If EEE has been detected near you, Dr. Watkins recommends doing your best to avoid mosquito bites and regularly wearing bug spray with DEET.

“It’s important to remember that this is very rare,” Dr. Adalja says. “EEE is not the norm, but if people have found it in your area, take action to prevent mosquito bites.”

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