The Zika virus (ZIKV) epidemic is spreading: 67 countries are now reporting transmission, and over 2,000 cases of congenital Zika syndrome (CZS) have been confirmed. The heaviest burden has been borne by those living where poverty, poor infrastructure, and lack of access to health services are common and the penetration of Aedes aegypti is high. Because most cases are asymptomatic, the most dramatic signs of the disease appear through the CZS cases. In spite of the need for disaggregated epidemiological data to understand transmission patterns and evaluate interventions in vulnerable populations, there is no reliable count of ZIKV cases by sex and ethnicity (1).