In the wake of this winter flu season, the holding of mass gathering, such as Kumbh Mela, may pose health risks. Maha Kumbh has been a case of study among health experts for a long time. Here’s what studies and research have to say about health challenges at Maha Kumbh.

Vast crowds of Hindu pilgrims in India bathed in sacred waters as the Kumbh Mela festival opened on Monday, with organisers expecting 400 million people -- the world's largest gathering of humanity -- to assemble over six weeks.India is the world's most populous nation, with 1.4 billion people, and so is used to large crowds.

Millions of Hindu pilgrims gathered on Tuesday at the confluence of the Ganges, Yamuna and Sarasvati rivers to take part in the first holy bath of the Kumbh Mela, the world's largest religious gathering.