It’s summertime so when the weather is fine many of us head outdoors. But there lurk mosquitoes, an all-too-familiar menace. What’s more, a wave of mosquito-borne tropical disease that first appeared in the Western Hemisphere in late 2013 has now spread across the Caribbean, stoking concerns about a debut in the continental U.S. The painful virus is already plaguing U.S tourists who have contracted it overseas, but it has not yet been transmitted by mosquitoes buzzing around the mainland. Last week, however, the first three cases were confirmed in Puerto Rico, expanding the disease’s reach.

Chikungunya (pronounced chik-un-GUNH-ya) first emerged last year on the Caribbean island of Saint Martin. As I reported in February, just a few months after the disease was first detected the number of cases had already climbed to more than 1,000 and had hop-scotched across several of the Caribbean islands. Since then, the disease has crept even closer to the continental U.S. with one confirmed case in the U.S. Virgin Islands and 14 cases in Haiti. There are now 4,576 confirmed cases of the disease in the Western Hemisphere, according to the Pan American Health Organization, and more than 165,000 suspected cases.

The disease is known for causing patients to stoop over from joint and muscle pain that typically lasts for weeks, but can persist for months or years. It can also cause high fever and headaches. The three cases in Puerto Rico were locally acquired and all occurred among patients who had no history of travel, says Brenda Rivera-Garcia, state epidemiologist for the Puerto Rico Department of Health. The first Puerto Rican patient started having symptoms in mid-May, she says.

Will the disease land in the U.S.? Epidemiologists still don’t know. Although more than 30 cases have been reported in the U.S. this year, all were among tourists returning from vacation in the Caribbean or Asia. Twenty-three of the cases were reported in Florida. Another four cases were in Virginia. A local outbreak would only begin if, for example, a mosquito in Florida bit an infected person, the virus incubated in the mosquito’s body for about 10 days and then that mosquito nipped other humans and successfully transmitted the virus. For now, the Centers for Disease Control and Prevention is carefully eyeing the number of travel-related cases. Those incidents will, “likely increase as the virus becomes more common in the region,” says Christine Pearson, a press officer for the National Center for Emerging and Zoonotic Infectious Diseases at CDC.

There is no vaccine for the virus and it rarely proves fatal. Still, since it emerged in the Western Hemisphere last year, there have been 14 deaths attributed to the virus.

See below for full list of recent cases: