Malaria is named after the wrong cause. The word comes from the Italian mal'aria, short for mala aria, "bad air." For centuries people blamed the fever on foul air rising from swamps and marshes.
The idea is very old. Hippocrates used the Greek miasma, "stain" or "pollution," for dangerous fumes from the ground. Rome suffered badly. A deadly form known as "Roman fever" struck the city and the surrounding Campagna again and again through its history.
The Italian phrase is old too. The Contagions history blog cites mal aere in a text on the Venetian lagoon from 1440. Etymonline says the physician Francisco Torti (1658-1741) probably first used mal'aria as a medical term.
English picked it up in 1740. That year Horace Walpole wrote from Italy: "There is a horrid thing called malaria, that comes to Rome every summer, and kills one." For Walpole, malaria was the thing in the air, not the illness. Etymonline records that it first meant unwholesome, contaminated air. "Malaria fever" appears by 1814. Only by 1866 was the word naming the disease itself.
Before that, English speakers called it the ague. The word comes from Latin (febris) acuta, "sharp fever." By the late 1300s it meant a fever with bouts of chills and shivering. It also went by marsh fever and paludism, both pointing to the swamps. Malaria gradually pushed these older names aside.
Then science took the theory apart. In 1880 the French physician Alphonse Laveran saw parasites inside the red blood cells of malaria patients. In 1897 Ronald Ross, a British doctor, proved that mosquitoes transmit the disease. Giovanni Battista Grassi showed that only Anopheles mosquitoes carry human malaria. Ross won the Nobel Prize in 1902, and Laveran in 1907.
Nobody renamed the disease. Miasma theory is long gone, but every mention of malaria still repeats it, in Italian.