In a drier world, some infectious diseases are unexpectedly thriving

A new review in Trends in Ecology and Evolution, surveying nearly 100 studies, finds drought can boost the spread of diseases like cholera, West Nile virus, and dengue even as it shrinks habitats for other pathogens, a pattern researchers call the 'drought-disease paradox.' Co-author Tara Stewart Merrill of the Cary Institute said the finding runs counter to the intuitive assumption that waterborne disease needs abundant water to thrive.

The mechanism often involves crowding: as ponds and watering holes shrink, animals and disease vectors cluster more tightly together, increasing contact and transmission opportunities. A 2017 drought in Somalia led to nearly 80,000 cholera cases and over a thousand deaths, and California droughts in the 2000s and 2014 drove excess valley fever and West Nile cases.

Severity matters too — some pathogens are eventually killed off by sufficiently harsh, hot, salty conditions — and researchers note that heavy rain preceding a drought can worsen subsequent outbreaks, a dynamic dubbed 'weather whiplash.' The paper's authors say better understanding these dynamics could help predict which diseases become bigger threats as droughts intensify with climate change.

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