About 60 percent of adult female bison in Yellowstone test positive for exposure to the Brucella bacteria. However, testing positive for exposure does not mean an animal can transmit the disease. For example, anyone who received a smallpox immunization will test positive for smallpox antibodies even though they are not infected with the disease and cannot transmit it. The number of infectious female bison that could transmit live bacteria varies from 10 to 15 percent of the population. There have been no documented cases of bison transmitting brucellosis directly to cattle, in part due to efforts by federal and state agencies to maintain separation between these animals. There have been many occasions where elk transmitted brucellosis to cattle. In both cases, transmission is possible and the likelihood increases during late winter when bison, elk, and cattle are more likely to share low elevation grasslands along the river valleys outside Yellowstone.