I don't think it's anybody's imagination. I am thinking back to the twenty some years I was living on antibiotics and every kid, including me, was a germ bag. It may have no correlation but we often are infected first by a virus before a bacterial infection takes hold. Antibiotics shouldn't impact immunity to fight other bacteria, however viruses are mortal enemies with bacteria and much of our bacterial conscripts aid to kill viruses that are pathogenic.
The course of antibiotics may kill off beneficial bacteria in the gut as well as every mucus membrane so ravaged by the infection antibiotics were given to eliminate. The antibiotic course was right to take as the bacteria was or would damage and scar the lungs or upper respiratory system. It is another good reason to consider allowing some infections to run their course when you, child and doctor think it's best.
This may be poor therapy for a kid but I take Zovirax often after a long course of antibiotics. In my case a common virus responsible for cold sores, herpes simplex, is pearled into my liver so I need no exposure, just an opportune environment. If a child can fight off even a few infections without antibiotics, it will potentiate into a more robust immunity. It's a thought.
LL
....Really?