For example, pilgrimage is motivated by different factors in different places. Some pilgrimage trips are made out of duty, while others are made in the hope of receiving special blessings or healing. It is important to distinguish between obligatory pilgrimage (as in the modern Islamic pilgrimage to Mecca) and pilgrimage that is a voluntary act involving a vow or promise (such as the sacred journey of the early Christians to Palestine or Rome). The obligatory pilgrimage inevitably involves larger numbers, guarantees the survival of the pilgrimage route and destinations, has its own intrinsic dynamics in all Islamic countries, and for one month a year in the city of Mecca in Saudi Arabia (with a resident population of around 150,000) has more visitors (over one million) than any other city in the world. As a result, the rapid spread of Islam was the major influence in making Islam the hub of a number of important trade routes, including caravan routes leading from the Middle East through Central Asia to northern China and across the Sahara up to Sudan. Many Muslim traders were also effective missionaries, acting as widely traveled multiple diffusion nuclei. Diffusion of expansion explains the spread of Islam from its area of ​​Arab origin, and diffusion of relocation explains its subsequent dispersal into Malaysia, Indonesia, South Africa, and the New World.