An emerging virus that is causing an unprecedented epidemic in Brazil and is quickly spreading through Latin America may be responsible for a spike in severe birth defects. The Zika virus, a little-known pathogen that until 2007 hadn't been seen outside of Africa and Asia, spread earlier this year to South America and has infected more than 84,000 people in Brazil. Zika usually causes relatively mild symptoms, including fever and rashes. Many infected people do not get sick at all.

But the Brazilian government is now warning that the virus may be responsible for a dramatic increase in cases of microcephaly, a severe birth defect in which the brain fails to develop properly and the head is much smaller than normal. Children with microcephaly frequently have developmental delays, learning disabilities, impaired motor function, and seizures. 

The connection is not yet proven, but if Zika virus is causing birth defects, it would be a serious public health issue. Transmitted by mosquitoes, the pathogen is rapidly expanding its range. In October and November, cases were identified in Colombia, Suriname, Guatemala, El Salvador, and Mexico. There is no vaccine or treatment for the little-studied virus, and its mosquito hosts are common as far north as the southern United States.

Microcephaly can be caused by genetic factors, infections, or injuries. In recent years, there have been between 150 and 200 cases in Brazil per year. As of 30 November, more than 1200 cases had been reported in 10 states, all of which have also reported Zika virus infections, says Ana Maria Bispo de Filippis, head of the flavivirus laboratory at the Oswaldo Cruz Institute in Rio de Janeiro, Brazil.