Given a choice between dengue fever and another mosquito-borne disease called chikungunya fever, most would choose dengue. Neither has an available vaccine or specific treatment, but chikungunya is far more debilitating.

The disease has long been a problem in Africa and southern Asia, causing high fever and severe joint pain. The name “chikungunya,” meaning roughly “that which bends up” in the Makonde language of southeastern Africa, describes the doubled-over posture of the afflicted.

Now the virus is drawing closer to the U.S. The World Health Organization recently reported the first outbreak of chikungunya in the Western Hemisphere, on the Caribbean island of St. Martin. As of early January, there were 99 confirmed cases on the island, as well as a smattering of cases on other Caribbean isles. The U.S. Centers for Disease Control and Prevention warns that the virus could spread to other islands and the surrounding mainland in the coming months or years. The flow of tourist traffic through the region also heightens the risk of an outbreak in the U.S.

Just how chikungunya reached St. Martin has not been determined. The earliest patients diagnosed had not recently left the island, so they most likely acquired the disease locally. One plausible explanation is that a traveler contracted the disease in another region of the world and brought it back to St. Martin, where a local mosquito spread the virus to others. (All it takes to spread chikungunya is for a female Aedes aegypti or Aedes albopictus mosquito to feed on an infected person's blood and then bite someone else.) Another, less likely, option is that an infected mosquito traveled to St. Martin, perhaps as a stowaway on a ship or plane.

“We know the area has the right mosquitoes to potentially transmit chikungunya, so you could question, ‘why not before now’ or ‘why not a year from now,’” says Erin Staples, an expert in vector-borne diseases at the cdc. “This just happened to be the right combination of factors.”