That's interesting.
Sounds like the "Columbian Exchange". Where Christopher Columbus came over in 1492 and "exchanged Eurpean goods for products of the two Americas". Starting a huge transfer of plants, animals, culture and diseases. The original native American populations were almost wiped out with about 90% dying of the (for them) new diseases, forced labour and warfare That was about 55 million people from the original 60 million !
The outcome isn't always negative, though. In the 1930s, Martians invaded Earth, and we were completely defenseless against them. Fortunately, the Martians succumbed to some of Earth's native bacteria and the invasion was thwarted. Unfortunately, they apparently came up with vaccines, came back in the 80s disguised as humans, and took over industry and politics. There was a guy who claimed he could see them when he put on these "magical" glasses who engaged in terrorist activities to stop them. However, the setback was temporary as they're back in full force in the world today, as is plain to see. Would have loved to see the World Cup trophy ceremony with a pair of those glasses! 😅
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.
Same thing happened to indigenous populations all over the world. Not just from the severe diseases either. Even the common cold took out many because they had no immunity. I believe it hit the Pacific Islanders pretty hard but also here in Australia.