El Niño will increase temperatures and make precipitation more volatile, which in turn could fuel the spread of pathogen-carrying mosquitoes, bacteria, and toxic algae. It’s a preview of the ways climate change will influence the spread of infectious diseases.

As with La Niña, the effects of an El Niño extend far beyond a patch of above-average warmth in the Pacific. Parched regions of the world — like Chile, Peru, Mexico, and the American Southwest — are often bombarded with rain and snow. Some other parts of the world, including the Northeastern U.S., the Amazon, and Southeast Asia’s tropical regions, on the other hand, don’t see much rain at all in an El Niño year. The planet could temporarily become 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer, on average, than in preindustrial times — a threshold scientists have long warned marks the difference between a tolerable environment and one that causes intense human suffering.

These patterns are a boon for certain vector-borne illnesses — defined as infections transmitted by an organism (usually an arthropod, a category that includes insects and arachnids). Regions of the world that will experience longer wet seasons because of El Niño, many of which are in the tropics, may see an increase in mosquito-borne illnesses, according to Victoria Keener, a senior research fellow at the East-West Center in Honolulu, Hawaii, and a coauthor of the U.S.’s upcoming Fifth National Climate Assessment. “El Niño will mean a longer breeding season for a lot of vectors and increased malaria potential in a lot of the world,” she said.

Vibrio cholerae, the water-borne bacteria that causes cholera, is another area of concern, experts told Grist — both in areas that see more rain during El Niño and those that see less rain. Flooding aids the spread of the cholera bacteria from open sewers and other waste containers — still prevalent in many underdeveloped parts of the world — into drinking water systems. Drought also leads to an uptick in cholera cases in poor countries, because restricted access to fresh water forces people to use less water for personal hygiene practices like handwashing and turn to unsafe sources of drinking water.

Poisonous algae is a consideration in regions where El Niño spurs above-average sea-surface temperatures. Algae thrive in warm water, where their poisons accumulate in water-filtering organisms such as shellfish.

Read the full article about El Niño driving infectious disease by Zoya Teirstein at Grist.