Each year, about a million people are killed by malaria. But even in countries where the disease takes a heavy toll, new research finds, the risk can vary: A subtle difference in a single gene can cut the risk of developing a life-threatening case of malaria by up to nearly 90% in people who have already been infected.

The gene mutation causes people to ratchet up production of nitric oxide (NO), a gas that plays a role in a variety of physiological processes. Previous studies with rodents found that NO can guard against malaria and other diseases, but no one was sure if it had the same protective effect in humans.

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