Typhus is the name for a number of diseases, caused by bacteria called Rickettsiae. These bacteria are parasites that cannot survive outside their host organism. Depending on the species the host organisms are, fleas (on rats), harvest mites (on several rodents and humans), or lice (on humans). When the term is commonly used, it usually means "endemic typhus", which is spread by lice. The name "typhus" comes from Greek, where it means "smoky" or "hazy". This was the word used to describe the state of mind the patients are in.