It seems like people with AI disease either get sick all the time or never get sick.
I'm like you. Even though I work in a school (germy children!), things that are "going around" tend to miss me. My doctor told me that since my immune system is hyper alert all the time (since it's working to fight off my poor thyroid, lacrimal glands, etc.), it also kills off things that would make me sick.
My lymph nodes, however, are often swollen. I've noticed it throughout my life, and now that I know I have AI disease, it finally makes sense.