Alphaviruses: Population genetics and determinants of emergence

► Alphaviruses are important causes of emerging diseases as well as biothreat agents. ► Venezuelan equine encephalitis causes equine-amplified epidemics and enzootic spillover. ► Eastern equine encephalitis virus is one of the most virulent viruses for people and equids. ► Chikungunya virus repeatedly emerged from Africa into epidemic urban cycles in Asia and Europe. ► Population genetic studies are needed to identify natural and intentional alphavirus emergences.