The Hajj is the largest annual gathering in the world. More than two million people from almost every country in the world participate in this spiritual pilgrimage to Mecca, Saudi Arabia.

Given the large number of people who participate in this gathering, there may be an increased risk of infectious diseases such as meningococcal disease, tuberculosis, flu, or influenza, and gastrointestinal infections. Travelers may also have an increased risk of heatstroke, sunstroke and physical injuries.

Source: http://www.phac-aspc.gc.ca/tmp-pmv/thn-csv/hajj-fra.php