The brain is the most “physical” part of the human body.
We can live without a spleen, gall bladder, urinary bladder, most of the intestinal tract and have our anus sewn together.
We can live with one kidney, a partial liver, one lung, no limbs, no gonads, no breasts, lymph system reduced.....but we can not live without a functioning brain stem unless on support.
Mental illness is a disease that can progress to death of the physical body.
Just like an insect bite can cause an illness that turns into plague and death. Just as a bacterial infection can lead to fatal meningitis or flu becomes fatal pneumonia.
PTSD or any mental illness can be just as lethal as any of the above through no fault of the victim.