Government researchers in Brazil are set to explore the country's peculiar distribution of Zika-linked microcephaly - babies born with abnormally small heads. Zika virus has spread throughout Brazil, but extremely high rates of microcephaly have been reported only in the country's northeast. Although evidence suggests that Zika can cause microcephaly, the clustering pattern hints that other environmental, socio-economic or biological factors could be at play. We suspect that something more than Zika virus is causing the high intensity and severity of cases," says Fatima Marinho, director of information and health analysis at Brazil's ministry of health. If that turns out to be true, it could change researchers' assessment of the risk that Zika poses. Zika was discovered in 1947 and hadn't been implicated in birth defects until now. The northeast was where the first reported surge in microcephaly cases in Brazil began a year ago. Health officials had expected that they would later see the same high rates in other parts of the country. But as of 20 July, almost 90% of the 1,709 confirmed cases of congenital microcephaly or birth defects of the central nervous system reported in Brazil since last November were in a relatively small area ... about the size of the United Kingdom, whereas Brazil is almost as large as the United States. There are many hypotheses about what might be going on. Marinho says that her team's data, submitted for publication, hint that socio-economic factors might be involved.