This was about correlation, not causation, but a study of more than 100,000 people concluded that the presence of a stress-related disorder (PTSD, acute stress disorder, etc) makes someone 30-40% more likely to also be diagnosed with an autoimmune disease (such as celiac disease, psoriasis, or rheumatoid arthritis).

I'd argue this number is an underestimation, considering some of us suffered with undiagnosed celiac disease for 30 years after our traumas, and the study was 30 years long.

Chronic and severe acute stress has been a known autoimmune trigger for a long time, but this is an unprecedented study in its size, length, and comprehensiveness.