I have seen an explanation somewhere, I've actually been trying to find it this morning as I'm interested in it again.
For me, I know I have a lot of immune system DEPLETION. My CD57 is under 20. My lymphocytes are also extremely low. So I have no idea why I am never getting sick. I used to get sick constantly. From what I read, it's very common for Lyme patients to not catch colds and flu's. Maybe the immune system can only handle so much at once.