Every year for a century, since Christian missionaries founded the hospital in 1906, the coming of the rainy season has marked the start of a desperate pilgrimage. Clouds gather; downpours erupt; mosquitoes hatch; malaria surges. There's no time to lose. Parents bundle up their sick children and make their way to Kalene hospital.

They come mostly on foot. Some walk for days. They follow trails across borders, into rivers, through brushwood. When they reach the hospital, each child's name is printed on a card and filed in a worn wooden box at the nurses' station. Florence, Elijah, Ashili. They come through the heat and the rain and the dead dark of the cloudy night. Purity, Watson, Miniva. Some unconscious, some screaming, some locked in seizure. Nelson, Japhious, Kukena. A few families with bicycles, Chinese-made one-speeds, the father at the pedals, the mother on the seat, the child propped between. Delifia, Fideli, Sylvester. They fill up every bed in the children's ward, and they fill up the floor, and they fill up the courtyard. Methyline, Milton, Christine. They pour out of the bush, exhausted and dirty and panicked. They come to the hospital. And the battle for survival begins.

From the mosquito's salivary glands to the host's liver cell: a quiet trip. Everything seems fine. Even the liver itself, that reddish sack of blood-filtering cells, shows no sign of trouble. It's only in those few rooms whose locks have been picked by falciparum where all is pandemonium. Inside these cells, the malaria parasites eat and multiply. They do this nonstop for about a week, until the cell's original contents have been entirely digested and it is bulging with parasites like a soup can gone bad. Each falciparum that entered the body has now replicated itself 40,000 times.

The cells explode. A riot of parasites is set loose in the bloodstream. Within 30 seconds, though, the parasites have again entered the safe houses of cells—this time, each has drilled into a red blood cell, flowing through the circulatory system. Over the next two days, the parasites continue to devour and proliferate stealthily. After they have consumed the invaded cells, they burst out again, and once more there is bedlam in the blood.

For the first time, the body realizes it has been ambushed. Headache and muscle pains are a sign that the immune system has been triggered. But if this is the victim's first bout of malaria, the immune response is mostly ineffective. The alarm has sounded, but the thieves are already under the bed: The parasites swiftly invade a new set of blood cells, and the sequence of reproduction and release continues.