A mass gathering is a planned or spontaneous event where the number of people attending could strain the planning and response resources of the community or country hosting the event. The Olympic Games, The Hajj, and another major sporting, religious, and cultural events are all examples of a mass gathering.

Mass gatherings may stretch health systems beyond their capacity; however, these events also present opportunities for long-lasting positive effects such as stronger public health systems after the event, or residents and visitors that are better informed about how they can protect themselves from certain diseases.

[WHO, 2008]