How do I increase my baby's immune system? He is 6 months old and falls sick at least once a month.

My baby's 6 months as well and she hasn't been ill eversince. I think it's because of the breastmilk. We even don't give her vitamins that the our Pedia gives. If you're baby is not breastfeed, I think it's best to consult your Doctor for prescrition and I believe fresh air and relaxing ambience will help. Stay away from city noises and pollution for at least twice a month.