When a new disease hits a virgin population, disaster usually results. The Bison came to North America between 150 and 200 thousand years ago over the Bering land bridge from Asia. The evolved from the bison in Eurasia as the steep bison. They thrived in the North American grasslands. The introduction of the European bovine which introduced Brucellosis almost wiped them out. By 1889 there were only 541 bison left. By conservation efforts in the population is back up to over 30,000 individuals. The conservation stated at Yellowstone National Park and Great Teton NP in Wyoming. Because of the fact they were almost wiped out, there was some hybridization by selective cross breeding cattle to produce more genetic variation. The herd in Yellowstone and Wind Cave national parks are the only herds that test pure. However, the cross hybridization seems to have worked to help the bison recover. Today bison are being introduced to other ares. In fact there are sufficient numbers that bison ranches are becoming frairly common in the Western US. In fact I would much rather have a bison steak than a beef steak. The mean is very lean and has more taste and less health issues to humans.
While bison are not out the woods they are doing much better. Both bison and cattle can get Brucellosis, today it is not a sure death to death to bison. Of course the Europeans also in introduced small pox to North America to native population that had never seen small pox and it decimated the native American population.