Why is my child sick again???

I thought I had done everything right, and maybe I had? I breastfed for 18 months, exclusively for the first 6 months, I feed my child very healthy foods (broccoli, fish, sweet potato, chicken soup, etc) and very few unhealthy foods (he is 19 months and has never had cake, cookies, ice cream, or any other sweet) and I even give him basic supplements (cod liver oil, vitamin D and probiotics). I even avoided Purell and other hand sanitizers because they supposedly either don't kill all the germs or create "super bugs" (I am still unclear). I make sure he is well rested, well hydrated and that he gets plenty of fresh air and exercise in addition to multiple hefty doses of "mommy love" each day. So why is my child sick again?