Hi Alexk,

I believe there can be a connection. I got my second concussion in a year in January, and got sick immediately afterwards, although I usually get sick less than once a year.

Less anecdotally, from my own internet research, concussions can affect the autonomic nervous system, which is responsible for regulating a variety of physiological systems, ranging from circulation (see concussion-induced exercise intolerance, POTS), digestion (IBS), to immune system depression.

Issues with the autonomic nervous system can be generalized as dysautonomia, and a bunch of information can be found in a review paper (google "Autonomic Dysfunction after Mild Traumatic Brain Injury" by Dmitry Esterov and Brian D. Greenwald), with immune system depression referenced in a separate paper.

In short, yes, there can be a connection.