Since the Zika virus was first identified in a monkey in Uganda in 1947, it has rarely caused illness in humans. When people have become infected with the virus, the disease it causes has generally been mild, with small outbreaks occurring in a relatively narrow geographical band around the equator. The situation today is dramatically different. Last year the virus was detected in the Americas, where it is now spreading explosively. Possible links with neurological complications and birth malformations have rapidly changed the risk profile of Zika from a mild threat to one of very serious proportions.