Disease surveillance during a large religious mass gathering in India: The Prayagraj Kumbh 2019 experience

•At Kumbh Mela 2019, disease surveillance was established for 22 acute diseases and syndromes.•Among the reported illnesses, 95% were communicable diseases such as acute respiratory illness (35%), acute fever (28%), and skin infections (18%).•The incident command centre generated 12 early warning signals from indicator-based and event-based surveillance: acute diarrheal diseases (n = 8, 66%), vector-borne diseases (n = 2, 16%), vaccine-preventable disease (n = 1, 8%), and thermal event (n = 1, 8%).•There were two outbreaks (acute gastroenteritis and chickenpox) that were investigated and controlled.•Implementation of disease surveillance facilitated early outbreak detection and response.