The Zika virus has spread rapidly across Latin America, transforming from a little-known, mild ailment to an international public health concern. The illness is transmitted through the Aedes aegypti mosquito and until recently was believed to only cause mild symptoms. There is now evidence linking Zika to a spike in cases of pregnant women giving birth to babies with microcephaly, a birth defect in which a baby is born with a smaller than normal head, and Guillain-Barre syndrome. The CDC expects the virus to continue to spread and has urged pregnant women to avoid the two dozen countries where the virus is present so far and to take extra precaution to avoid mosquito bites.