Not human diarrhea, but...one of my cats has chronic diarrhea which we have 't been able to resolve, and he tends to "leak."
He sleeps with me, so I end up washing my bedding every day. I use the soak cycle, my regular Aldi laundry detergent, and cold water, and that has always done the trick - no stains. Of course, since I wash the bedding daily, stains don't have time to set. (That's really the secret with stains - washing promptly.)
I use the soak cycle on almost all my laundry. I get pretty grubby, and the soak cycle dies a good job. The only things on which I use special treatment are kitchen dishcloths, which I wash hot and with bleach; blood stains, which I pre-treat with hydrogen peroxide; ink stains, which I pre-treat with hairspray; and oil stains, which I pre-treat with a stain remover.