The spread of Zika is part of an unnerving trend: Several mosquito-borne tropical illnesses have lately been spreading into regions of the world that have never experienced them.
A viral disease called chikungunya — which had never appeared in the Western Hemisphere until 2013 — has lately affected Central and South America, even making an appearance in Florida last year. (Its name comes from the Makonde language of Tanzania, where it was discovered in 1952; it means "that which bends up," referring to the contorted physique of a person afflicted by the virus.)
Dengue fever, known as "breakbone fever," has also seen new outbreaks in Puerto Rico, Florida, Gulf Coast states, and Hawaii — all places that hadn't usually been affected. In 2015, Brazil reported nearly 1.6 million dengue cases, a big increase from 569,000 in 2014.
Zika, dengue, and chikungunya are all spread by a species of mosquito called Aedes (in particular the Aedes aegypti and Aedes albopictus mosquitoes). For reasons researchers don't understand, these mosquitoes have been more effective at bringing diseases to new places lately, affecting fresh populations that don't yet have the antibodies to fight off the viruses.
Heidi Brown, a professor of epidemiology at the University of Arizona, explained there are at least three factors that help these illnesses spread: the number of mosquitoes out there, the number that are biting humans infected with the virus, and the number that are surviving long enough to infect other humans.
"The survival of the mosquito is driven a lot by temperature," she added. Mosquitoes thrive in warm and moist environments. "So people go to the idea of global warming — that climate change and changes in precipitation patterns and temperature are helping mosquitoes survive in different areas." In other words, warming is helping expand the range of places that are habitable to mosquitoes.
There are other factors that may be driving the trend, too: People are traveling more than ever, bringing diseases to new locales. More and more people live in crowded cities, where it's easy for viruses to jump from person to person and for mosquitoes to find large concentrations of humans to feast on.