It is said that kindergarten teachers get sick allllll of the time their first few years of teaching. After that they rarely get sick at all.
As a mother of 5, I rarely get very sick any more.

I don't know if TBIs/PCS affect immune system, but regardless, it is important to keep your immune system healthy. I drink a ton of water (carry a quart canning jar with me and drink from that). I also eat very heathy vitamin rich foods (nettles and dandelions right now -- very high in VERY good stuff for you). Play in the dirt a lot, so that your body is used to various bacterias and can build natural immunity. And wash your hands often -- especially when around children. (I also drink green tea daily and use elderberry syrup when I feel that first twinge of illness).

I think it has to do with our levels of energy which are low. The immune system then does not have as much energy to protect us or heal us.