Inside the Hajj, One of Earth's Largest Gatherings

One of the largest gatherings on Earth, the annual hajj began Thursday and continues until October 7. During that period, an estimated two million Muslims will participate in the pilgrimage.

Making the journey at least once in a lifetime is a requirement for all Muslims who are able-bodied and have the financial means to go. The word "hajj" means "to intend a journey," and refers to the physical trip as well as to spiritual intentions.

During the event, pilgrims visit a number of holy sites to pray and participate in sacred rites. (See "Bright New Mecca Train Ferries Hajj Pilgrims.")

This year, the hajj has been marked by increased security concerns due to unrest in the Middle East, the Middle East Respiratory Syndrome (MERS) virus, and the ongoing Ebola crisis in West Africa. Saudi Arabia has banned people from the three countries hardest hit by the Ebola epidemic-Sierra Leone, Liberia, and Guinea-from entering the country.