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Over the next few weeks officials at the World Health Organization (WHO) face a tough and politically charged call. The Muslim month of fasting, Ramadan, begins July 9 and could draw as many as two million people from around the globe to the holy sites of Saudi Arabia in a pilgrimage called umrah. But a new disease, called Middle Eastern respiratory syndrome, or MERS, could threaten them.Infectious disease control at mass gatherings is always a challenge, but this year even more so. Saudi Arabia is currently waging battle with MERS, yet it has released only the barest of details that scientists or public health officials could use to try to prevent its spread within Saudi Arabia or around the globe. In early May Saudi officials startled the world by announcing 13 new cases over the course of a few days. Since the start of May there have been 38 new cases worldwide—31 of them in Saudi Arabia—and 20 of the victims have died. With virtually no clues to draw on about where the virus lives in nature and how people contract it, WHO is trying to figure out what guidance to give those pilgrims, and the countries they will return to, about how to avoid infection and the international dissemination of a devastating new illness.

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