Brucellosis is a bacterial disease caused by Brucella abortus. Infections cause female ungulates to abort their calves and produce "undulant fever" in humans. Efforts to eliminate brucellosis in livestock began in 1934 and for a brief time in early 2008, cattle in all 50 states were considered free of the disease. Before the disease was controlled in livestock, however, it spread to bison and elk in the Greater Yellowstone ecosystem, the last place where brucellosis remains endemic in the United States.