Recurrent viral infections. Awareness.

Hi, My son is 22 months old and wieght is around 12kg.. from last one year when he was 11 months old.. he has been catching various kind of viral fevers, seasonal flu almost every or alternate month. I have been keeping very strong and close checks on his eating habits and the things he should eat for good health but not able to prevent him getting into viral fevers.. I am wondering what can be the issue because when we got his blood test checks done on his last to last viral fever, nothing unusual appeared. Can anyone please put some light if it is normal for infants, babies or toddlers to catch viral fevers everytime or if it can be the impact of area we are living. We are staying in Ghaziabad, NCR region and it is known to be one of the most polluted cities. P.S. - Doctors please ignore my question if you are looking for clients. Will appreciate the doctors who are interested and spend their precious time in raising awareness. Thankyou in advance