It is of major concern and often times the MA community overlooks this in training. Bad guys are just that...bad guys. And they often spend or have spent time in a correctional facility and/or a mental health care facility. The incident of exposures in these types of facilities are many times higher than 'normal life'. So the bad guy attacking you...does he have HIV? Hep C? TB? It is a little like playing Russian roulette. He could be carrying nothing but a bad attitude. You could pull off a head punch that puts him down and saves the day. But what if he is carrying something that will alter the course of the rest of your life? I've known people in that position and to say it sucks is a little bit of an understatement that people can't fully appreciate unless they've personally experienced it or know those that have. And safer options fortunately exist in training that are just as effective. Nothing is 100% of course, but I try to load the dice in my favor as much and as often as possible.
That is one of the primary reasons we 'punchers' do thousands of knuckle pushups and other knuckle conditioning exercises over the years and build up callouses to reduce the chances of the skin breaking.