Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times. They were also part of the natural and cultural history of Jackson Hole, Wyoming, for thousands of years before they were extirpated in the 19th century as a result of commercial, subsistence, and sport hunting, and competition with domestic livestock. Present-day Jackson bison are descendants of 32 bison relocated from Yellowstone and Theodore Roosevelt national parks in the 1940s and 1960s, respectively, and some subsequent migrants from Yellowstone. The largest bison population in the country on public land resides in Yellowstone, and the Jackson herd is now the second largest unfenced bison herd in the U.S. Both populations are among the few that do not contain cattle genes as a result of interbreeding. However, some of these bison are infected with brucellosis, a livestock disease that has been transmitted to wild bison and elk through contact with infected fetal tissue or afterbirth. Access to winter range outside the parks has become a key issue because of limited tolerance for bison due to brucellosis and the risk of property damage and human injury.