zico was still a major health concern not so long ago and its effects remain. steve harrigan. >> five years ago it was a different virus, zeke, it made headlines and it was spread by tropical mosquitoes or contact for adults and children, the symptoms were mild, a rash, fever, 80% of those infected were asymptomatic. it had a devastating effect on the brain of a growing fetus, causing microcephaly, abnormally small heads, underdeveloped brains. >> very much like covid is now, it was a new virus we'd not been exposed to before. as the virus came in, there was a lot of susceptible population. >> by 2016 there were more than 650,000 cases worldwide. 3,700 birth defects. in the u.s. alone, there were more than 5,000 cases. south florida began intensive mosquito fumigation. the cdc spent $85 million on preparedness and response, standing up zika labs. they issued a warning for pregnant women to avoid travel to 22 countries. just as suddenly as it appeared, the number of zika cases fell dramatically. experts believe herd immunity developed, halting the spread before