Until the nineteenth century, medical writers rarely failed to note the influence of *grief, despair, or discouragement on the onset and outcome of illness, nor did they ignore the healing effects of faith, confidence, and peace of mind.
スラッシュはカンマの後、前置詞の前に入れます。
Until the nineteenth century, /medical writers rarely failed to note the influence of *grief, despair, or discouragement /on the onset and outcome