That's a touchy subject, especiallyon Montana ranches that border Yellowstone. The cure for it is erradication. Any bison wandering off of park lands into Montana can be killed immediately.
From an article:
Brucellosis has been eradicated in most of the United States except for several major sources in free-living elk and bison in the Greater Yellowstone Area and adjacent land. In a very controversial proposal, the National Park Service has set a goal of eliminating brucellosis in bison and elk in the Yellowstone area by 2010. The Park Service’s efforts, along with the State of Montana’s policy of killing any bison that stray from the park area, have resulted in severe reduction of the U.S. bison herd at Yellowstone.

There is no harm to humans though. I don't think it's a sprevelent as it used to be. But the bison herd is noticably smaller.