When a 16-year-old boy showed up at a north central Florida urgent care center in August of 2016, no one could figure out what he was infected with.

According to a report recently published on the case in the journal Clinical Infectious Diseases, the boy (who remains anonymous) had a fever of around 100 degrees Fahrenheit (38 degrees Celsius).

A rash that started on his chest was spreading to his abdomen, arm, back, and face. It was aggravated by heat and sunlight, though didn't cause pain.

The boy said that he'd been bitten by numerous mosquitoes while attending band camp.

The case appeared in the midst of the Zika outbreak, but the teen tested negative for Zika, Chikungunya, and dengue.

However, in one of the urine samples collected by doctors, researchers eventually identified a virus that's been known to infect animals, including squirrels, raccoons, and whitetail deer: Keystone virus.

This was the first time that a Keystone virus infection has been confirmed in a human, though it's known to be widespread among animals in the southeastern US, from the Chesapeake Bay to Texas.

The disease comes from a virus family that's known to cause encephalitis, or brain inflammation. And it's a reminder that there are always emerging diseases to be watching out for.

Diagnosing the Keystone virus

Identifying the infection was no easy task, since there'd previously been no way to test for Keystone virus. But because of the Zika outbreak happening at the time, researchers were determined to identify the condition to see whether there was reason to be concerned about more new diseases being spread by mosquitoes.

"We couldn't identity what was going on," Dr. Glenn Morris, director of the Emerging Pathogens Institute at the University of Florida, told WUSF Public Media. 

"We screened this with all the standard approaches and it literally took a year and a half of sort of dogged laboratory work to figure out what this virus was."

It's possible that Keystone virus is more widespread amongst people than experts realise, according to the case report.

Surveys conducted almost 50 years ago found antibodies to the virus in about 20 percent of people in the Southeast, indicating that they'd been exposed, though live virus had never been found in a person before.

Most likely, any symptoms of the virus that people have experienced have been mild, like fever or rash.

There are no reported symptoms in animals, though in some areas, 30 percent of squirrels or 10 percent of deer surveyed have been found to be infected with Keystone virus.

Researchers are most concerned by the fact that Keystone virus is part of the "California serogroup" family of viruses, which are known to cause encephalitis, or brain swelling, that can be dangerous.

It's possible that Keystone could cause this in some cases, based on observations of the virus in cell cultures and the behaviour of related viruses. But that didn't happen to they boy in the case report.

As the researchers wrote, doctors should start looking for Keystone virus in cases when patients have unexplained viral encephalitis.

"It's one of these instances where if you don't know to look for something, you don't find it," Morris said in a statement.

Plus, they wrote, this finding underscores the fact that there are all kinds of diseases circulating out there that could one day infect humans.

This article was originally published by Business Insider.

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