They probably all caught it from different sources, and remember that some people are careless with hand washing too! So if you always wash your hands as soon as you come home and always before meals and touching your face, you should be fine. As long as they don't come into your home touching everything, you will be alright. Also, if it's not noro (less severe) it's not as contagious either, so it'll be more difficult to spread around. The real danger is really if someone gets sick inside your home and even then lots of people have managed to stay illness free even in that case. I was careful throughout college, wouldn't touch food with my hands when outside of my home (except when drunk.. silly me) and I'm doing my master's now, and I haven't (yet) caught anything from talking to people who are sick. Just make it clear to your friends that they are NOT to touch your things after they have been ill and to always wash their hands when they visit you. You should be fine that way.