Cases of a deadly mosquito-borne virus called EEE have prompted health officials in Michigan to urge the public to cancel or reschedule outdoor events after dusk, especially if those events include children.

The urgent appeal comes at a time when many schools and youth groups typically hold evening practices for marching band, football, cross country, field hockey and other activities that would be difficult to move indoors.

"Our winters are long, so people want to be outside and enjoy the warm weather in the evenings," said Edward Walker, a professor of microbiology and entomology at Michigan State University.

"Unfortunately, right now that's a risk," he warned.

The request to stay inside is only for residents of certain counties in the southern half of the state, where seven human cases of EEE, short for eastern equine encephalitis, have been reported this season. Three of those patients have died.

"Michigan is currently experiencing its worst eastern equine encephalitis outbreak in more than a decade," Dr. Joneigh Khaldun, chief medical executive for the Michigan Department of Health and Human Services, said in a statement.

Michigan's high case count is not an outlier: 2019 appears to be the worst year for EEE cases in more than a decade across the nation.

On average, seven cases a year are reported to the Centers for Disease Control and Prevention, but this year, at least 20 people have been diagnosed with EEE in Connecticut, Massachusetts, New Jersey and Rhode Island, as well as Michigan.

Only 2 percent to 6 percent of people who are bitten by mosquitoes carrying the virus go on to develop EEE. But in those that do, the illness hits hard and fast.

Symptoms such as headache, high fever, chills and vomiting start four to 10 days after a person is bitten. As the disease progresses, the patient can experience disorientation, seizures and coma.

Inflammation of the brain leads to death in about one-third of cases, and people who do survive are often left with brain damage.

Gregg McChesney died of EEE in August. Family photo

One of those who died suddenly from EEE this year was Gregg McChesney, 64, of Kalamazoo County, Michigan.

"Within a matter of nine days, he went from perfectly healthy to brain dead," McChesney's brother, Mark McChesney, told NBC affiliate WOOD-TV. "He had a seizure and next thing you know, he's in the ER and he just never came out of it."

Lab tests later confirmed McChesney died of EEE.

The mosquito

The main carrier of the virus that causes EEE is a species of mosquito called Culiseta melanura, which feeds on birds, spreading the virus to them.

Another mosquito species, Coquillettidia perturbans, feeds on these infected birds, as well as on horses and humans. That's how people become infected.

Those mosquitoes are particular about where they live and breed, favoring red maple or white cedar swampy areas of states that border the Atlantic, Gulf and Great Lakes.

The sphagnum bogs found in southern Michigan are also a favorite for mosquitoes that spread EEE.

"We normally see this mosquito come out in Michigan in late June, early July, then tail off and disappear," said Walker. "The interesting thing about this year is that we still have good numbers of them persisting into mid-September."

The mosquitoes are expected to stick around until the first hard frost.

Some research has suggested that high levels of rainfall may contribute to the problem, especially in the swampy wetlands where mosquitoes like to hide.

But Walker said there is no good model for predicting EEE outbreaks, and it's unclear why 2019 has been a bad year. His research suggests other outbreaks will occur sooner rather than later.

"My review of historical records shows that the period between EEE outbreak years is getting shorter, and we’re having more frequent ones," he told NBC News.

The CDC says the best way to avoid EEE is to avoid mosquito bites. That means using an EPA-approved mosquito repellent, wearing long sleeves and pants in wooded, swampy areas, and getting rid of standing water in flower pots, bird baths and anything else in your yard that holds water.

Follow NBC HEALTH on Twitter & Facebook.