Since 1964, when it was first identified, Keystone virus was thought to transmit only from mosquitoes to wildlife such as deer and raccoons. Now, doctors have confirmation that it has infected at least one human: a teenage boy from Florida tested positive for presence of the pathogen, researchers reported earlier this month in Clinical Infectious Diseases.

Coauthor Glenn Morris, the director of the Emerging Pathogens Institute at the University of Florida, says it’s likely a fairly common, and in many cases harmless, pathogen among people. “There’s a high likelihood that Keystone virus is continuing to circulate in Florida, as it has for at least 50 or 60 years, and as it may well have for hundreds of years,” Morris tells WUSF. “This is one of our native Florida viruses.”

For decades, infectious disease researchers have suspected Keystone virus infects humans. A study from the 1970s revealed that...

The recent case was found during a screening campaign for Zika virus in Florida in 2016. The boy had developed a fever and rash and sought medical attention. But it took a year and a half of laboratory work to determine that Keystone virus was present.

Keystone virus is a member of what’s known as California serotype viruses, which can cause brain inflammation, or encephalitis. Although the research team found that it can infect mouse brain cells in culture, the boy did not develop encephalitis.

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