So tired of my kids being sick!!

I am soo frustrated and starting to feel like I am just a horrible mom! It seems like my kids are always sick! Especially Zander! Ever since August it seems like he constantly has a cold. He will have about two weeks of feeling better and than he is sick again! I do not know what I am doing wrong... I make sure he eats either fruits or vegetables with every meal and he definitely gets plenty to eat. I always dress him warm and when we go out he always wears a coat and hat. When he gets over a cold I wash all of his bedding, sanitize all of his toys and everything he may have touched and still he gets sick over and over again! This last time he was better for 5 days and now his nose is way yucky again. Any one have any advice?? It's so weird.. for the first 10 months of his life he was hardly ever sick and now he spends more time sick than healthy. Thank goodness he is not prone to ear infections or we would really be in trouble!