I've had a couple kids like this, where other parents were getting frustrated with it. I just enforced my sick policy with everyone. The one child's mom cried (tears and all) about how she was so frustrated and annoyed that she had to keep picking him up or keeping him home. I just said that all the other parents felt the same as her but they were keeping their child home until they were 100% and that maybe she should too. It was a brutal month or more with illness because kids were coming back before they should have.
The other child was a drooler and always sick. I've never cleaned so much in my life!! Lol. I ended up terming for other reasons and a couple parents said "maybe the others won't get sick so much now"..... There really isn't much you can do other than clean! I tell the parents about any illness going around so they can keep their eyes open for early symptoms and hopefully decrease the spreading of said illness