The genetic link is key to the prevailing theory about Crohns -- that it is an overactive immune response to one or more triggers or irritants, caused by a faulty gene. In some patients the runaway immune response (which causes the chronic inflammation that is the hallmark of the disease) is limited to the wall of the GI tract. In some others it can be a more general response, hence the Crohns-related arthritis, skin conditions, etc. that those patients get. But it does not make sense that an overactive immune system, even if it is general rather than specific to the gut, would make one more susceptible to other illnesses. Iin fact, it ought to be the reverse, and it seems to be that way with me. Rarely get colds or flu type illnesses. Aside from a heightened immune response in my small gut and terminal ileum, with the trouble that causes, I am in overall good health and have been for the 40-some years I have had this disease.
Of course this is the prevailing theory . . . which seems logical in light of the causes and genetic roots of a lot of other autoimmune diseases. It remains to be absolutelyproven for Crohns, but I buy it and most doctors and patients do too.