Health officials warn travelers coming back from Cuba about Oropouche virus.

A virus called Oropouche, also known as sloth fever, has infected over 20 people returning to the U.S. from Cuba. The virus is spread by small biting flies and mosquitoes in tropical areas.

Oropouche was first discovered in 1955 in a forest worker and was named after a nearby village. Scientists initially believed sloths played a role in spreading the virus, hence the name sloth fever.

While the virus is typically found in forested regions, it has spread to towns and cities. Although human travel is believed to contribute to its spread, person-to-person transmission has not been observed.

Starting last year, large outbreaks of Oropouche occurred in South America and the Caribbean, with around 8,000 cases reported. The U.S. Centers for Disease Control and Prevention confirmed 21 cases in the U.S., all linked to travel to Cuba.

Symptoms of Oropouche resemble those of other tropical illnesses like dengue and Zika, including fever, headaches, and muscle aches. Currently, there are no vaccines or specific treatments for Oropouche infection.