Origins
Malaria is a word built on a mistake. It comes from Italian mal'aria — a contraction of mala aria, meaning 'bad air'. For centuries, Europeans believed that the fevers plaguing swampy regions were caused by noxious vapours rising from stagnant water.
The theory made intuitive sense: malaria was most common near marshes, worst in summer, and struck at night when damp mist hung low. The connection between swamps and sickness was real — but the mechanism was wrong. It wasn't the air that was dangerous, but the mosquitoes that bred in standing water.
The word entered English from Italian around 1740, decades before the true cause was discovered. In 1880, the French physician Charles Louis Alphonse Laveran identified the Plasmodium parasite in the blood of malaria patients. In 1897, Ronald Ross proved that mosquitoes transmitted it.
Later History
By then, 'malaria' was too deeply embedded in every European language to replace. The word survives as a linguistic fossil — preserving a theory that was wrong but seemed so obviously right that it shaped the vocabulary of medicine.
The Latin root malus ('bad') also gives us malice, malady, malaise, and malevolent. In each case, the ancient Roman concept of badness echoes through modern English.