The ENSO effect: Climate variability, disease ecology, and implications for mass gathering events

The El Niño–Southern Oscillation (ENSO) is the most powerful interannual climate driver influencing the global distribution of temperature, rainfall, and humidity. Its alternating El Niño (warm) and La Niña (cold) phases shape infectious disease dynamics by altering vector ecology, water sanitation, and human exposure. ENSO has been associated with outbreaks of dengue, malaria, cholera, Rift Valley fever (RVF), leptospirosis, encephalitides, and hantavirus infections across multiple continents. In the era of mass gatherings (MGs) such as the Hajj and Umrah pilgrimages, the Kumbh Mela , international sporting tournaments, and global tourism events ENSO-driven climatic extremes pose amplified health risks. Understanding ENSO’s mechanisms and predictability offers unique opportunities to integrate climate forecasting into epidemic early warning systems, safeguarding the health of millions of mobile participants. This review explores the pathways linking ENSO and infectious disease transmission, with special emphasis on MG settings. It argues that climate-informed preparedness must become a central component of MG medicine to anticipate, rather than merely respond to, future epidemics.