A spate of tropical diseases never before seen in the United States are gaining a foothold here, in part because climate change lets mosquitoes and ticks expand their ranges, according to new research.

The New York Times reports the list of scary bug-borne illnesses grows longer every year: Lyme, West Nile, Chagas, dengue, chikungunya  and now Zika, the first case of which turned up in Puerto Rico last week.

What’s driving the spread of such diseases, scientists say, is hotter weather, cheap airfares that allow for more international travel, and crowding in tropical cities that allow for the rise of infectious disease.

Brazil has been hit hard by Zika, a mosquito-borne virus that may be causing thousands of babies to be born with tiny heads and brains, a defect called microcephaly.

Until May, Zika had never touched this hemisphere except on Easter Island, 2,200 miles off the Chilean coast. Now it circulates in 14 Latin American or Caribbean countries and Puerto Rico.

Last month, the Centers for Disease Control and Prevention predicted Zika would follow the pattern in the United States that dengue has: many cases in Puerto Rico, followed by outbreaks in Florida, Gulf Coast states and maybe Hawaii.