Why does malaria make fevers come back every two or three days?

Because the parasite multiplies inside red blood cells on a cycle of two or three days. After an infected Anopheles mosquito bites, the parasites first reproduce and mature in the liver without causing symptoms. A few days later they spread into the bloodstream, where they invade red blood cells and multiply inside them. After 2 or 3 days, each infected cell bursts, releasing 16 to 32 new parasites along with waste and fragments of the cell, and that release triggers fever. The new parasites invade fresh red blood cells and the cycle repeats, so without treatment the fever can settle into a regular pattern, coming back every two or three days.

That rhythm was noticed long before anyone knew what caused it. Hippocrates, in ancient Greece, already classified these fevers by their beat, calling them tertian, quartan or quotidian depending on how often they struck. The disease's modern name comes from an even older guess: medieval Italians called it mala aria, 'bad air', blaming the fevers on swamp vapors rather than the mosquitoes that actually bred there. It took until 1880 for a French army doctor, Charles Laveran, to see the real culprit, a parasite living inside red blood cells, and until 1897 for a Scottish physician, Ronald Ross, after four years of collaboration with Patrick Manson, to prove the parasite's complete life cycle in mosquitoes. Working in a hospital in Calcutta, he showed that certain mosquitoes transmit malaria to birds, which proved that the mosquito was the vector for malaria in humans.