My parents visit us monthly and get colds pretty much every time they visit us. My in laws visit every three months. They bring a cold every time and swap colds with the kids.
This Christmas my mom got norovirus from one of the grandkids (who got it from mine). After that my parents decided to stay away for the rest of winter. I'm not sure when we'll see them again. I think that one crossed the line.
Do your parents and ILs get a lot of germs from LOs? How do they handle it?