A group of acute infectious and contagious diseases, caused by rickettsiae (bacteria) that are transmitted by arthropods (fleas, ticks, mites, and lice), and occurring in two principal forms: epidemic typhus and endemic (murine) typhus. Also called "jail, camp", or "ship fever".

The word typhus comes from Greek typhos, "smoke, vapor, vanity, conceit", and was used in early English from Late Latin typhus, to mean "pride".

Dropped in the 17th century, it was revived (or taken afresh from the Greek) at the end of the 18th century to mean "a fever that clouded the mind".

The more familiar typhoid fever, and like typhus; it was long supposed to be a form of the other disease; but it is now more accurately called enteric fever—enteric, from Greek enerikos, from enteron, intestine, “inner”.

Epidemic typhus is considered a particularly prevalent disease where unsanitary conditions exist. It often develops on shipboard, in army camps, and where living conditions are unfavorable and where there is an over congestion of people. The disease has been considered rare in the U.S.

The onset of symptoms is sudden: severe headache, pain in the back and limbs, and extreme prostration occur.

Fever rises rapidly, often reaching 104 to 105 degrees F (40 to 40.6 degrees C) in two to three days, remains high for about ten days; and then falls by the time of the crisis.

The pulse is rapid, weak, and the tongue is tremulous and may be covered with a whitish fur; in severe cases, it becomes black and rolled up like a ball in the back of the mouth.

The patient demonstrates stupor, delirium, muscle twitching, and picking at the bedclothes.