Lots of people will suggest things you can do to not become sick. My rheumatologist's absolute favourite is getting the flu jab (no, I've still yet to get it). Others will recommend bathing in hand sanitiser, avoiding contact with people completely and living in your house until spring. However, I have one tip that will absolutely guarantee that you won't get sick:
Don't order lemon.
When you're out to eat and you ask for a glass of water, make sure to ask for no lemon. Lemon wedges are notorious for being one of the bacteria filled things you can find in a restaurant. This is usually since they are kept in an unrefrigerated container all day and touched by many different people repeatedly. And no, the citrus does not keep bacteria at bay. If you have a suppressed immune system or cannot risk being sick whatsoever (or both), don't bother with the lemon. Ask for another glass of water or request a dish not come with lemon. Trust me.
The other night I went to a restaurant and had asked for a glass of water. It had a lemon in it, which I didn't realise it would come with. I knew it was likely to be dirty but I was too shy to ask for a new glass of water without lemon. So I drank it anyway, reassuring myself it wouldn't get me sick. The next morning I woke up sick. Right now I'm laid up with a fever, cough and pounding head.
Do as I say, not as I do. Learn from my mistakes, my children.