AIBU to NOT buy into the belief that exposing toddlers to every bug going is good for their immune system? Nurseries are terrible places for bugs and so many DCs spend half their time with D&V, colds, fevers, rashes etc. If you want an example of the 'constant cough/cold syndrome' look on Childrens Health!
True some parents have no choice and children do need to see other children. But they dont do much interactive play before the age of 3 and their immune system has plenty of time to build with more limited contact with other kids and adults.
My own DCs went only to playgroup weekly before school started, and they didnt collapse under the bombardment of viruses when they did go, just the usual minor stuff. My DD never had an antibiotic til she was a teenager!
Please dont bang on about having no choice due to work (you obviously don't have a choice) but there are plenty of mums who do have the choice and choose to send babies and toddlers to nursery.