"One. That is the total number of locally transmitted Zika cases confirmed in the continental United States this year, as of mid-August. That single case, recorded on 26 July in Hidalgo County in Texas, which borders Mexico, contrasts with hundreds of cases of local transmission last year.

Better control of Zika's vector, the Aedes aegypti mosquito that thrives in the hotter, southern part of the country, doesn't explain the dearth of cases. Nor are other factors such as climate change at work, experts say. Instead, Zika cases have plummeted in Latin America and the Caribbean, where the virus raged over the past 2 years, and much of the population is now immune to it. That, in turn, means fewer infected people entering the United States, reducing the chances of mosquitoes spreading the virus to susceptible people. The respite, experts say, could last for years.

Zika, a member of the flavivirus family, has circulated in Africa and Asia for decades but had never been seen in the Americas before it surfaced in Brazil in 2015. It had been considered relatively harmless, but in February 2016 the World Health Organization (WHO) in Geneva, Switzerland, declared a Public Health Emergency of International Concern after compelling evidence from Brazil began to link the virus to devastating brain damage in thousands of babies whose mothers were infected while they were pregnant. Still, infectious disease experts expected that, like most mosquito-borne flaviviruses—which include dengue, West Nile, and yellow fever—it would initially tear through the population but then fade into the background as people developed immunity."

Jon Cohen reports for Science August 16, 2017. [2]