ZIKA, a mosquito-borne virus that arrived in Brazil last May, is an avid traveller—and an increasingly feared guest. It has since found its way into 21 other countries in the Americas. Until October, Zika was not thought much of a threat: only a fifth of infected people fall ill, usually with just mild fever, rash, joint aches and red eyes. Since then, though, evidence has been piling up that it may cause birth defects in children and neurological problems in adults. El Salvador, Colombia and Ecuador have recommended that women delay pregnancy until 2018. America’s Centres for Disease Control and Prevention (CDC) has advised pregnant women against travelling to countries where Zika is circulating.

The virus was first identified in 1947. Since then it has caused small and sporadic outbreaks in parts of Africa and South-East Asia. In Brazil, for reasons yet unclear, it quickly flared into an epidemic after its arrival—by official estimates infecting as many as 1.5m people. Alarm bells started ringing in October, when doctors in Pernambuco, one of Brazil’s north-eastern states, saw a huge increase in babies born with microcephaly: an abnormally small head, often with consequent brain damage. In the next four months more than 3,500 cases of microcephaly were reported in Brazil. That compared with fewer than 200 a year in the five previous years.

The spread of Zika makes attacking disease-carrying mosquitoes all the more important. Mostly, Zika is transmitted by Aedes aegypti, which is also the vector of dengue and yellow fever. This insect lives in tropical climes, but Aedes albopictus, found as far north as New York and Chicago, and in parts of southern Europe, can also do the job (though it is not clear how efficiently). A paper published last week in the Lancet shows where Zika could become endemic (see map). But places where air-conditioning, screened windows and mosquito control are the norm are unlikely to see outbreaks flare up. Researchers in America and other countries have begun work on a vaccine. Unlike the one for Ebola, though, which had been in the pipeline for a decade when the epidemic in West Africa began, a Zika vaccine is “at ground zero” says Alan Barrett of the University of Texas.

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