Purely anecdotal here, but my youngest daughter is 6 and has always been sick over the winter months with a cold, runny nose, cough, sore throat, etc since she was a baby. Not for a few days, but for weeks straight every cold season, she just never seems to shake it. We think she has maybe a weaker immune system as my other daughter almost never gets sick (she is 8).
This cold season so far--not even a runny nose for one day, no cough, sore throat, fever-- nothing at all. It's been pretty remarkable. Kids have been in school 4 days a week most of the year here.