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). It is an epidemic disease.
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.
Typhus can be treated with antibiotics.