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 17 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. On January 15th America’s Centres for Disease Control and Prevention (CDC) advised pregnant women not to travel to countries where Zika is circulating. The virus was first isolated in 1947, from a monkey in the Zika forest in Uganda. 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 years before 2015. None of the known causes of the condition—which include genetic abnormalities, drugs, alcohol, rubella infection and exposure to some chemicals during pregnancy—seemed a plausible culprit.

Last week, CDC scientists announced the best evidence so far that Zika can pass from mother to fetus: they found the virus in four Brazilian babies with microcephaly who had died in the womb or shortly after birth. Previously, Brazilian researchers had found Zika in the amniotic fluid of women carrying fetuses with microcephaly.

There is another fear. After Zika arrived in Brazil, and also in El Salvador, both saw a sharp increase in severe neurological and autoimmune problems, including Guillain-Barré syndrome, which can lead to paralysis. These also surged in French Polynesia after Zika broke out there in 2013.

Working out the extent to which Zika, alone or combined with other things, is to blame for any of this is tricky. Dengue and chikungunya—mosquito-borne viruses with similar symptoms—are common where Zika is making the rounds. According to Scott Weaver of the University of Texas, tests that spot Zika work only during its infectious phase, which lasts just a few days. After that, they are often useless if the patient has had dengue or been vaccinated against yellow fever. And only laboratories that can do sophisticated molecular tests are in the game in the first place. All of which means that most cases of Zika are missed, and many are misdiagnosed.

Bearing these caveats in mind, researchers are mining the available surveillance data for answers. More solid results will come from prospective studies, set up recently, which are tracking pregnant women in Brazil, looking at whether those who catch Zika are more likely to have babies with birth defects.

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, also of the University of Texas. That is where potential antiviral drugs are, too.

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.

In December, Brazil decreed a national public-health emergency. This has removed bureaucratic hurdles to the purchase of insecticides for mosquito larvae, equipment for health workers and the like—and prompted speculation about whether this bureaucracy was necessary in the first place. It also enabled the deployment of the army to help 310,000 health workers in the mosquito-eradication drive. Brazil was declared free of A. aegypti in 1958, after a campaign that included regular fumigation and visits to ensure households got rid of standing water, where mosquitoes like to breed. Since then, the insect has bounced back. Might the fear of Zika help finish the job properly this time?