"Tropical diseases — some of them never before seen in the United States — are marching northward as climate change lets mosquitoes and ticks expand their ranges.

But that does not mean that epidemics will break out, scientists say. Whether a few cases explode into a full-fledged outbreak depends on a set of factors far more complex than the weather.

The list of scary bug-borne illnesses seems to get longer every year: Lyme, West Nile, Chagas, dengue, chikungunya — and now Zika, the first case of which turned up in Puerto Rico last week.

Donald G. McNeil Jr. reports for the New York Times January 4, 2016. [2]