Most of human feces is bacteria. Billions upon billions of tiny bacteria corpses. No, wait, they don’t all have to be dead. Billions of bacteria make up your shit. Diarrheal or not.
When I am torturing medical students around the open body (substitutes for campfire) I like to ask them, “Tell me in simple words that any layman could understand what the small bowel is for, then the large bowel.” After they struggle through arcane and complex answers I tell them what I was looking for. (This Socratic method of teaching in medicine is known as “pimping”, by the way.) The small bowel absorbs the calories out of your food. Every single calorie you are ever going to absorb. All the protein, fat, carbs, minerals, vitamins (except one). All the essential nutrients. Everything out of a GNC bottle that’s actually going to be of some use to you comes in through the small bowel.
Then what is the large bowel there for? It dries the shit.
No kidding. When the chyme (digested food byproducts, but not yet shit - generally odorless, or has a faint smell of food) exits the terminal ileum and enters the cecum, it is a continuous stream of slightly pasty fluid. I tell the students, “If you need an operation that gives you a colostomy, that is, you have to ‘wear the bag’, and if you are unlucky enough to need it in your right lower quadrant” (putting my hand on the spot), "you have to keep the bag on 24 hours a day. You have to use special sticky sealants to be sure the bag is attached and not becoming detached by the icky stuff flowing out. You have a bag of liquidy stuff hanging off you all the time.
“On the other hand, if you need a colostomy in the usual place, over here” (hand on the left loewr quadrant), “then you can put the bag on in the morning after coffee, expel your turds, take it off, shower, put on your shirt, and go to work. You can make love with your spouse without it on. You can put it on when you need to take a dump.”
This is only true if you aren’t having diarrhea.
Diarrhea is not a symptom which is of use to the body - in fact, it’s highly dangerous if it gets bad. Cholera is essentially an attack of diarrhea (plus vomiting) so bad it kills you in a day or two. Diarrhea kills children in the third world because it dehydrates them and robs them of electrolytes like potassium that are lost with the watery feces; that’s why a simple rehydrating solution saves so many lives.
Diarrhea means that the function of your colon, which is to dry the shit, is being interrupted. Bacteria (almost always; rarely viruses; occasionally amoebas or other parasites) are making your colon sick, and it’s unable to dry the shit. In some cases (like the vibrio of cholera, which makes a toxin which directly affects colonic mucosa cells), the colon turns around and actually pours water out, stealing it from your bloodstream via the capillaries in its wall.
Now if you take a microscope down to the colonic mucosa, better still if you take a scientist down with you, you will see that “It dries the shit” means a thousand ingenious and tiny little operations involving miniature potassium and sodium transporters in the membranes of the cells, to suck out these electrolytes and let water diffuse passively after them. Billions of bacteria hang out against these mucosal cells, sometimes helping them actively (E. coli makes vitamin K which we can reabsorb - the only vitamin not absorbed in the small bowel), sometimes helping them passively (great volumes of E. coli may block more dangerous bacteria like shigella and salmonella from reaching the mucosa), sometimes harming them actively (E. coli O157:H7, in uncooked hamburger meat, has killed people’s children). All these things are involved in a huge production line, or dance if you prefer, which ends us up with neat little turds instead of a continuous stream of pasty shit. And any large and complex production line can go wrong. Think of ordinary diarrhea as workers throwing their wooden shoes into the machinery they resent, and cholera diarrhea from a vibriotoxin as terrorists taking down the factory with sarin gas. If the factory recovers, the water extraction process begins again, the water goes back into the venous capillaries that drain your gut, and you end up with neat little turds again.
A final remark. Why do we have this large system to dry the shit? After all, it’s expensive to the body in terms of blood supply, nutrients required, constant maintenance (colonic mucosa cells die and are replaced faster than any other cell in the body), and the presence of a huge reservoir of internal bacteria that can be remarkably dangerous to us.
My answer to this question comes from no Cite but makes a little sense. The only large animals in Nature that have continuous pasty dribbles of shit are fish and birds. These animals are hard for large predators to track by the dribble of shit. Let’s you and me start on the savannah at five million years ago. You have no colon, and I’ll have a colon. There’s a sabertooth cat on our trail. Which of us do you think it’ll pick off first?