They're more likely to get a high number of colds and flu, strep throat, infectious hepatitis and spinal meningitis, as well as other diseases. Until recently it was thought that this was OK because babies were merely building up immunities they'd have to build up later anyway, when beginning preschool or kindergarten. But studies have found that delaying group care makes sense. Even when first placed in a group setting, older children have lower rates of illness and disease because children are more resistant as they get older. In addition, the way babies and toddlers interact makes transmission of illness more likely.
Babies drool, suck fingers and then touch teachers, toys and each other.
Diaper changing also carries risk of transmitting infection. Babies are also simply more vulnerable to disease. These illnesses spread out into the community, creating more sick adults as well. Then, the adults would get sick, too, and had to try not to infect co-workers. The episode with pink eye was awful.