More than 2 million Muslim pilgrims descended on Mecca this week for the annual hajj, one of the five pillars of Islam. Diseases carried by travelers are always a concern, and this year Saudi officials took precautions against Ebola. In addition to screening all arriving airline passengers, the Saudi Ministry of Health refused to issue visas to would-be pilgrims from countries that have been hit hardest by Ebola: Liberia, Sierra Leone, and Guinea.