Climate change is the defining issue for the 21st century. Climate variables affect the air people breathe, the water they drink, the food they eat, and even where they are able to live. Extreme weather events are becoming the norm and records are constantly being broken, with the past three years ranking as the hottest since records began. For infectious diseases, climate change is a threat multiplier. It takes existing threats – whether from a cholera outbreak, the spread of Zika to new geographical areas, or the severe malnutrition that accompanies drought – and enhances them. The risks are familiar but their impact is amplified in frequency and severity. A changing climate can expand the distribution of infectious diseases, especially those transmitted by mosquitoes and other vectors, and invite the emergence of others. The emergence of Nipah virus and Hanta virus as human pathogens has been traced to extreme weather events that forced animal hosts to leave their ecological niches and invade human settlements.