diphtheria

Where it comes from

From French diphtérie (later for a while also spelled diphthérie) in 1857, which was coined in 1855 with the suffix -ie to replace -ite in the previous term diphtérite for the disease because it affects more than one part of the body. The previous term (later for a while also spelled diphthérite) was coined in 1817 by Pierre Bretonneau using Ancient Greek διφθέρα (diphthéra, “prepared hide, leather”) in reference to the tough membrane that forms in the throat.

Bretonneau perhaps coined and used the Latin term diphtheritis (with its close imitation of Greek spelling typical of Neolatin) even earlier than the French term diphtérite, which follows the French habit of sometimes spelling t for the Latin transliteration th of Greek θ (but not, for example, in thermomètre).