we had this problem with a group I used to belong to. One toddler was always coming down with sicky, cold bugs and infecting the others - some of whom were teeny babies. Unaccepotable IMO.
In the end, I sent an email to the group asking that mums ask the group's permission to bring along a toddler with a bug. Needless to say, the mum in question went bananas at me. However, she calmed down a bit when I explained it was unfair on the tiny babies plus, most of us had babies that still weren't sleeping through and sickness was an added burden that, for me, made life intolerable. Given that her DD rarely woke in the night, even when sick, she kind of needed this spelling out to her.