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When a malaria outbreak erupted in Malaysian Borneo in 2002, researchers were surprised to find that the culprit wasn’t Plasmodium malariae, the main mosquito-borne parasite known to infect humans in the area. Instead, the parasite’s DNA turned out to stem from P. knowlesi, colloquially known as “monkey malaria,” which is specialized to infect and proliferate in forest-dwelling macaques. A few accidental cases had been recorded in people over the years, but such an outbreak was unusual. And it didn’t stop there: P. knowlesi has since become the most common cause of malaria in Malaysia, and human infections are steadily rising throughout Southeast Asia.
It’s one of several instances of vector-borne pathogens that have popped up in humans in areas that are undergoing widespread deforestation. The forests of Borneo are being felled at a rapid rate, foremost to make way for palm oil plantations. Researchers have long ...