Typhus is any of several similar diseases caused by Rickettsia bacteria. The name comes from the Greek typhos (τύφος) meaning smoky or hazy, describing the state of mind of those affected with typhus. The causative organism Rickettsia is an obligate intracellular parasitic bacterium that cannot survive for long outside living cells. It is transmitted to humans via external parasites such as lice, fleas and ticks. While "typhoid" means "typhus-like", typhus and typhoid fever are distinct diseases caused by different genera of bacteria.