Zika is a mosquito-borne virus that was first identified in Uganda in 1947 in rhesus monkeys, and in humans in 1952 in Uganda and the United Republic of Tanzania. The Zika virus is transmitted to people through the bite of an infected mosquito (Aedes aegypti in tropical regions). This is the same mosquito that transmits yellow fever, dengue, and chikungunya. Sexual transmission of Zika virus has also been described. Outbreaks of Zika virus disease have been recorded in Africa, the Americas, Asia and the Pacific.

Health authorities have observed an increase in Guillain-BarrĂ© syndrome (a potentially fatal neurological condition that can cause paralysis) and an increase in babies born with microcephaly (a birth defect characterized by abnormally small heads associated with severe cognitive disorders, intellectual disability, and possibly death). These coincide with Zika virus infections in the general public. Scientists believe Zika can potentially spread into the brain of a fetus through a pregnant woman’s amniotic fluid, attacking the neural cells and stunting the baby’s brain development. In the first months of the Zika outbreak, the number of reported cases of microcephaly increased 20-fold in Brazil. More investigation is needed to better understand the relationship between microcephaly in babies and the Zika virus. Other potential causes are also being investigated.

On Feb 1, 2016, WHO declared the cluster of microcephaly cases and other neurological disorders as a health emergency. The Zika infection is spreading rapidly globally. There is currently no vaccine available and a number of new vaccines are under development. In order to respond to the global epidemic, the GAIA consortium was able to start developing a globally standardized case definition of microcephaly (along with the definitions of other key obstetric and neonatal outcomes that are being developed). All the definitions will be applicable in developed and low and medium income countries.