At the end of October, the Brazilian state of Pernambuco, located in the northeast of the country, started reporting a dramatic increase in the number of babies born with microcephaly, a condition characterized by a shrunken head and brain. The cases in Pernambuco were soon followed by similar ones in the nearby states of Bahia and Paraiba.

The cases of microcephaly seemed to be connected to an outbreak of the Zika virus, first recognized in Uganda in the 1940s. Zika is transmitted by Aedes aegypti, a species of mosquito that thrives in stagnant water pools. In the same way as malaria, dengue and Chikungunya, the Zika virus infection results in similar symptoms: fever, joint pain and rashes.

Thus, ominously, parallel to the increased number of pregnant women infected with the Zika virus there was an increase of microcephaly cases in their children. Because CT scans of those babies' brains showed signs of calcification usually associated with an infectious disease rather than a genetic abnormality, many experts thought that their microcephaly was caused by the Zika virus.