Rickettsia is one of a group ol disease-producing organisms. They are generally smaller than bacteria. When rickettsiae are introduced into the blood stream of human beings by bites of ticks or lice, they cause diseases such as Rocky Mountain spotted fever, typhus, and dengue. Rickettsiae resemble viruses, because they cannot survive outside of living cells. They are named for Howard Taylor Ricketts (1871-1910), an American pathologist.