Learn about Zika, a mosquito-borne virus that hit Brazil in 2015. Over the last few decades, diseases that spill over from animals to humans have been on the rise. Soon after the onset of Zika, epidemiologists noticed an uptick in the number of babies born with microcephaly, a severe birth defect affecting the head. Were the two related? See how epidemiologists studied the unfolding outbreak to determine Zika's origins, and also to determine if the virus was responsible for the babies born with microcephaly.