Microcephaly is a serious birth defect characterized by an abnormally small head. In Brazil, this congenital malformation is typically rare—only about 150 babies among the nearly 3 million born in the country each year are diagnosed with microcephaly. However, since October 2015, close to 4,000 cases of microcephaly were reported in Brazil, and many of the cases come from the northeast corner of the country.
In response to the dramatic rise in the malformed small heads, the World Health Organization (WHO) declared a public health emergency (something the agency has done three times since 2007) to get scientists and global resources focused on the problem.
The cause of the rise in microcephaly remains unknown. While a handful of possible culprits have been proposed, from the beginning officials have turned all their attention to a single suspect: the Zika virus.
Until recently, Zika was an obscure mosquito-borne illness considered to have relatively mild symptoms compared to other mosquito-transmitted diseases such as malaria, West Nile, and dengue fever. It was first identified in a rhesus monkey in the Zika forest near Entebbe, Uganda, in 1947, and up until a decade ago there were fewer than 15 documented cases of people who had contracted the virus.
