Muslims from around the world are gathering in Mecca for the annual hajj or pilgrimage. Around 3 million are expected, including more than 14,000 Americans. While diseases spread by travelers are always a challenge during the hajj, this year health officials are especially concerned about a deadly virus called the Middle East Respiratory Syndrome or MERS. The virus has already killed dozens of people in Saudi Arabia. Saudi officials are providing vaccinations. American pilgrims are being urged to take extra precautions.