Logic

I have never formally studied logic. It's quite complicated and beyond me. But common sense logic..yes one does appreciate that and I believed that I was quite good at that.
However, in that too, surprisingly often, I am proved wrong.
One such thing was autoimmune diseases.
I thought -- if one is afflicted with some autoimmune disorders, at least one will not succumb to common ailments...you won't catch a cold every third week..etc... efficient immune system isn't it?
The angle that I never thought about is this...with autoimmune diseases, the immune system is overloaded trying to fight its own body that it is quite inefficient when it comes to repelling externl attacks. Double disaster!!