G.K.
If telling everyone what the doctor's instructions were doesn't work, FIB.
My son (now almost 18) was born with what was termed "a depressed immune system". If someone even thought about having a bug & looked at him from a mile away, I swear he'd catch it. When he was really young & it was time for a family gathering (we have a really large family), I would make my apologies with the excuse that one of us wasn't feeling well & that we didn't want to share. We would then get together with the host at a different time if the get together was related to a birthday or special company. After a couple of years, it caught on and people quit taking their sick children to gatherings.