This post, written by Kai Kupferschmidt, originally appeared in Science Magazine.
Around the world, cholera is on the march. In Yemen, which is mired in a civil war, the devastating waterborne illness has sickened more than a million people since October 2016 and continues to spread. Outbreaks are ravaging Zambia, Tanzania, Zimbabwe, and several other African countries. Haiti is still suffering from an epidemic that began in 2010. And last month, after torrential rains flooded parts of Kinshasa, the capital of the Democratic Republic of the Congo (DRC), cases spiked there. "For reasons we do not understand, cholera seems to go through cycles of severe seasons," says David Sack, an infectious disease expert at Johns Hopkins University's Bloomberg School of Public Health in Baltimore, Maryland.