Apparently you've never heard of politicians. Slightly more seriously, the notion is that a lot of the more benign parasites secrete chemicals that relax the immune system. Since we evolved alongside the parasites, without their influence our natural immune response is too strong and we end up with autoimmune diseases.
That idea is still mostly unproven, but research is being done.
While peeling my potatoes, i came up with an idea that fits sort of to your idea :
Let say we have a person with a genetic defect that causes auto immune diseases. That defect could be a defect in coding for the autoimmune regulator AIRE. Making it less effective. That person starts a family and passes the genetic defect on to the offspring. Now this as a whole develops into a family. In this family, people do not get old because of a given autoimmune disease. For whatever reason, one of the offspring , let's say the daughter gets infected by a parasite that also regulates the immune system, dumbing it down. She seems to be a lot less affected by the genetically caused autoimmune disease.
Now the daughter gets married, has a child and while passing on the genetic defect, she also passes on the parasite. Voilá , the child seems to be auto immune disease free. The child grow up, get married and starts a family.
Continuing the passing on of the defect and the parasite. As long as the parasite is there, the autoimmune disease seems to be gone or a lot less problematic.