A mass gathering (MG) is an event, such as the Olympics, the World Cup, and the Hajj, which draws many thousands of individuals from around the world to a single location for some period of time. They can increase the geographic spread of infectious diseases as newly infected travelers return home. The goal of our research is to create a computational MG model to better understand the spread of infectious diseases and to use this understanding to develop new disease prevention strategies. Specifically, we are modeling the Hajj in which over two million Muslims from more than 189 countries travel to Makkah, Saudi Arabia annually.