Typhus is any of several similar iseases caused by louse-borne bacteria. The name comes from the Greek typhos, meaning smoky or lazy, describing the state of mind of those affected with typhus. The causative organism Rickettsia is an obligate parasite and cannot live long outside living cells. Rickettsia is endemic in rodent hosts, including mice and rats, and spreads to humans through mites, fleas and body lice. The arthropod vector flourishes under conditions of poor hygiene, such as those found in prisons, concentration camps, or refugee camps, amongst the homeless, or until the middle of the 20th century, in armies in the field. In tropical countries, typhus is often mistaken for dengue fever.