Kids in day care
can have a new cold every month.
Ultimately this is a good thing (so long as they're otherwise healthy) as the broad exposure to different viruses and "germs" helps their immune system to develop as it should - robust and able to handle lots of stuff later on. Kids who are raised in "sterile" surroundings often end up having problems like asthma
and lots of allergies
Answer assisted by David Miller, Medical Student