I assume you mean well, but this is serious “confidently incorrect” energy. Yersinia pestis, the bacteria that causes bubonic plague, never changed to become less virulent and can still affect humans to this day. It has been killing a ton of humans for thousands of years and was still killing thousands of people at a time in localized outbreaks up until we discovered the antibiotics that cure it.

Also, it’s transmitted through the fleas on small mammals, not through the mammals themselves. Flea transmission is far and away the primary vector. Human to human transmission has always been pretty rare, since it can only be transmitted between humans through contact with bodily fluids, similar to how HIV spreads.