“Stress” is an overused & easily confused word, medically speaking. There are several forms, quite different from each other.
Physical Stress — Running a marathon is physical stress; that’s usually healthy. Pain from anything is also physically stressful; it’s not healthy per se, but won’t do physical harm in and of itself. But many diseases or conditions like heart attacks, major injuries, significant dehydration, and many more can compromise the immune system, leading to increased risk of infections, blood clots, and other complications.
Emotional Stress — This can be due to an upcoming school exam, a break-up, etc. If motivating, it’s good; if unhappy, bad. These types of stress might cause symptoms like headache or fatigue, but won’t cause actual harm. However, Anxiety, Depression, PTSD, and other such conditions are considered frank diseases. There’s some evidence they might somehow affect the immune system or predispose to certain illnesses.
I see many patients with a variety of symptoms which they attribute to “stress.” They may be right; however, if their symptom is actually due to a real illness (physical or mental), they’d certainly benefit from diagnosis and treatment. So maybe “stress” is causing a symptom, but maybe disease is causing the stress!