Chikungunya is a scary-sounding virus with some scary symptoms: joint pain so excruciating that patients often can’t stand or even sit upright for months. The mosquito-borne virus got its start thousands of years ago in southeastern Africa, where it generally caused a slow but steady stream of cases. About 50 years ago a mild strain of the virus spread to Asia. Then, following a drought in Kenya in 2004, cases of chikungunya in Africa soared and spread eastward across the Indian Ocean, causing severe disease and affecting hundreds of thousands of people across Asia.

This new strain of chikungunya is apparently replacing the older, milder strains that previously circulated in Asia. But how? In a study published in the Proceedings of the National Academy of Sciences USA , researchers have figured out that as it traveled, the virus picked up a single mutation that allowed it to be transmitted much more efficiently by Asian mosquitoes.