Not only are they for ill people they are for ill strangers. Speaking generally, All of us carry around our own pet zoo of bacteria and so on and we mostly have the same sort of zoo as people we are close with.....the further away the people we mix with come from, the less of our zoos will overlap and the more likely we are to get an illness from a visiting bug. Its why the kiddies bring home noo bugs when they start playgroup and the whole family get them, also why the first term away at college can be carnage. Once all the bugs have been swapped, the zoos all settle down again. Of course in hospitals there is the added complication of all the challenges to the patients’ immune systems from the ops, illnesses and so on that they are there for. When there were still big mental health long stay hospitals, it used to be said that each one had its own brand of d and v and new staff would catch it then become immune. My non research experience makes me belive this was true.