A rare and sometimes lethal form of malaria usually found only in monkeys is becoming so common in Malaysia that it accounts for most malaria hospitalizations there, scientists have found.

In the Malaysian part of the island of Borneo, a parasite called Plasmodium knowlesi causes severe malaria three times as often as Plasmodium falciparum, which has long been considered the deadliest form of the disease. The finding was presented by Balbir Singh, the director of malaria research at the University of Malaysia in Sarawak, at the annual meeting of the American Society of Tropical Medicine and Hygiene last week in New Orleans.

P. knowlesi is normally found only in long-tailed and pigtailed macaques, who get only mildly ill. But as loggers and palm oil producers have pushed deeper into Malaysia’s forests, more humans have been bitten by mosquitoes carrying it.

Malariologists had believed that mosquitoes must pick up P. knowlesi from a macaque before they could transmit it to a human, so transmission outside monkey habitats would be impossible. But recent research in Vietnam raises the possibility that mosquitoes can pick it up from humans, meaning urban areas could also have epidemics.