Keystone virus: a virus first isolated in 1964 from mosquitoes in Keystone, Florida that has recently been described as being isolated from a human host. The virus is known to infect animal populations in coastal regions from Texas to the Chesapeake Bay and is carried by the Aedes atlanticus mosquito. Despite evidence that humans make antibodies to the virus, it has never been isolated from a human host until now. In June 2018 a report identified that the virus was identified from laboratory samples taken from a teenager who became sick in Florida during the 2016 Zika virus outbreak.
Keystone virus belongs to a group of viruses (arboviruses) that have been shown to cause encephalitis (inflammation of the brain) in humans, but the affected teen only developed a fever and rash without signs of encephalitis. Until now, there has not been a way to test for the virus, it is unknown how common the infection is among humans.
REFERENCE: Lednicky et al. Keystone Virus Isolated from a Florida Teenager with Rash and Subjective Fever: Another Endemic Arbovirus in the Southeastern United States? Clinical Infectious Diseases. June 09, 2018.