In 2020, the Journal of the American Medical Association (JAMA) described a retrospective study conducted in Sweden of 2,244,193 people diagnosed with stress-related disorders like posttraumatic stress disorder (PTSD) and their connection to impaired educational performance and general cognition.

Besides the subjects, the study also looked at siblings of individuals with stress-related disorders and over a million age-matched controls. Among the autoimmune diseases that were found were Crohn’s disease, lupus, and rheumatoid arthritis.

A 2018 retrospective study published in JAMA showed an increased risk for developing autoimmune disease in people suffering from PTSD. Before these prominent studies, several smaller studies offered proof that the immune system was directly affected by stress but nothing of this scale.

The connection most likely happens through your body’s chemical messengers and mediators that are hormones.

Levels of hormones in the blood, particularly corticosteroid hormones, change with stress and are the likely agents of change and gene expression during stressful events, but sex hormones might also be involved. The data are convincing.

That said, I and many others have believed this intuitively for decades. Most of us will never suffer from PTSD, but experienced early in life, stress can cause long-lasting changes in physiology and behavior.

Stress lowers our immune system’s resistance and opens us up to various infections through immunosuppression, specifically through increased corticosteroids, which are potent immunosuppressants.

The biological reason for this immunosuppression is the influence of the nervous and endocrine systems on the immune system that leads to inflammation, a condition that results in pain, fever, redness, and feelings of being unwell accompanied by loss of appetite, excessive fatigue, and/or sleeplessness.