It really doesn't bother anybody. But once the rats come into contact with humans, then the fleas are going to jump on the humans and then we have ourselves an epidemic because that's at the heart of bubonic plague. Because once the flea jumps on the human, it's going to enter the lymph node of where it's going into. That's going to create these huge swellings on the armpits, on the groins, on the lymph nodes of the human body. And then not long after, you're going to die. You're eventually going to get really high fevers, you're going to throw up, you're going to be at the shivers, you're going to be delirious, you're eventually going to get gangrene which is going to cause part of your body to turn black, hence the name black death. Now you're starting to understand.