Zika is a viral infection that usually causes a mild illness that typically lasts between 2 to 7 days. Zika is spread by mosquitoes. Eighty percent of people who become infected by Zika virus have no symptoms. However infection with Zika virus has been strongly linked with a serious birth condition called microcephaly. Microcephaly means a baby born with an unusually small head. In these cases, the baby’s brain may not have formed properly during pregnancy. Cases of a neurological illness (called Guillain Barre Syndrome) possibly caused by Zika virus infection are also being studied.