Risk areas for August (IMAGE) Umea University Caption Risk areas for August. Reproductive number (R0) explains the estimated epidemic growth rate of Zika. At levels below 1 epidemics die out. At higher values the epidemic grows exponentially. Credit <i>EBioMedicine</i> Usage Restrictions This photo may only be used in contexts related to this news release. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.