My yoga teacher always says this is the most important part of yoga. She says it's why she rarely gets sick. She was delighted to hear it was one of the top stories on HN.
It's possible that you're just not coming into contact with a lot of viruses (maybe the same for the yoga teacher).
Funny story of a friend of mine: had no kids, and started dating a pre-school teacher. First year they dated, he was sick 50% of the time.