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I’m curious if you think this is worth an addendum: During my last two years of UG, along with taking upper level chemistry courses (my major), I also began getting terrible symptoms throughout my body. I saw several doctors, and none could figure out what was going on nor treat me. Only now, 3-4 years later, have I been diagnosed with an extremely rare autoimmune disease, and have begun proper treatment. The disease + lack of grade inflation of my courses explain my relatively lower GPA (~3.2). *prior to this, my GPA was ~3.8* Addendum?

Assuming you can tie your condition to the lower grades (it sounds like you can, but you mentioned other factors), then yes, this is worth an addendum.

Don't bring up grade deflation or anything else besides your illness - it looks like you're trying to make excuses. Simply explain the symptoms, the diagnosis, and explain that the disease had an impact on your grades.