It does happen and is considered one of the risks of the job. That's why part of our training is to assess the situation FIRST, before even walking in. If it looks dicey, call for backup or wait till the rest of the crew arrives. You can't protect yourself all the time, but you can reduce the risk. I've been spat on, yelled at, had people try to claw/punch/kick me. It can be scary being a first responder.