We, along with our partners OpenDream, Epitrack, and InSTEDD, are hosting an EpiHack in Rio to explore the potential for participatory disease surveillance at mass gatherings. By letting people report on their own symptoms, can we get a better handle on disease outbreaks at big events like the World Cup, music festivals, the Olympics and more? We helped support a pilot project for mass gathering surveillance for the World Cup in Brazil last year. This week in Rio, hackers and public health officials from 11 countries around the world will work to develop concepts and, potentially, a prototype application for participatory surveillance for mass gatherings generally. You can track developments via the hashtag #hackforhealth, or following our partners twitter feeds at @epitrack, @opendream, @instedd, or @epihack.