When DS was a baby and we discussed daycare with his doctor, one thing that was mentioned was making sure the baby room was separate from the "germy toddlers" and preschoolers, which they do.
Any other group -- babies, older kids would probably be fine, but as the parent of a preschooler -- bleah! His preschool/daycare works with the kids to make sure they cover their mouths & noses, wash hands frequently, but they're still learning.