Our daughter has been at nursery 3 days a week since 1 year old.
For the first two years, between September - April, she was unwell with a cold every 3 weeks. We'd have 7-10 days of her being grotty with a cold, then a few weeks well, then the cycle resumed. We had to test for covid during much of that so we can see the more than monthly covid test results as proof it's not an exaggeration! Most were just colds or covid, but she's also had scarlett fever, flu norovirus, hand foot & mouth etc.
This winter (she's now 4) it was definitely better, more like every 4 weeks and she brushes off some of them in a few days, which makes a huge difference. Last 3 months have been horrendous in terms of viral illnesses but I think that's been the case with everyone, including adults.
The above doesn't seem to be unusual when we compare to friends and colleagues with nursery aged children. Some children seem to brush off colds etc even from an early age but if they don't and are sensitive to the runny nose & coughing and disrupted sleep etc it's more of an issue.
Nurseries are cesspits essentially. There's often 3-4 signs up on the nursery door each day over winter stating which infectious diseases children have gone home with recently!
My understanding is that it may limit the onslaught of colds when they start school though.