Ugggh I have one of those parents here too.
Constant snidey remarks about their child being sick again - same family who has no backup care so I understand the frustration.
Like the others though, I disinfect weekly (run all toys through the dishwasher and lysol wipe all toys that I can't) and I have said to her "I sanitize and colds and sicknesses happen which is why I don't charge for sick days so people will leave their sick children at home, there is only so much that I can do, I am just as frustrated as you are about this" and it seems to quiet her down.