epidemic typhus, a severe acute (sudden-onset) infectious disease with prolonged high fevercintractable headache, and a pink-to-red raised rash. The cause is a microorganism called rickettsia prowazekii. It is found worldwide and is transmitted by lice. The lice become infected on typhus patients and transmit illness to other people. The mortality increases with age and over half of untreated persons age 50 or more die. Also called european, classic, or louse-borne typhus.