A teen in Florida is the first known person to be infected by a mosquito-borne virus that previously had only been found in animals, Live Science reported Friday.

The Keystone virus was first discovered in 1964 in the Tampa Bay area and doctors had thought the virus could possibly affect humans. The teen, who went to an urgent care clinic in North Central Florida in 2016, was diagnosed after seeking help for a fever and rash.

The teen's symptoms were mild, but researchers said the virus could cause more severe symptoms including brain infections and may be more common among humans than previously believed.

"Although the virus has never previously been found in humans, the infection may actually be fairly common in North Florida," Dr. J. Glenn Morris, director of the U.F. Emerging Pathogens Institute, said in a statement. "It's one of these instances where if you don't know to look for something, you don't find it."

Two other viruses akin to the Keystone virus, Jamestown Canyon virus and La Crosse encephalitis virus, can cause inflammation of the brain, known as encephalitis.

The Keystone virus was not immediately identified in the teen, as doctors first believed it could be the Zika virus, but tests for that were negative. Researchers point to the need for more study into mosquito-borne diseases.

"All sorts of viruses are being transmitted by mosquitoes, yet we don't fully understand the rate of disease transmission," Morris said.

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