Everything about our bodies keep microorganisms out, wax in ur ears, tears in ur eyes, acidic skin..u get the point, the only real way we interact with the world is thru our mouth. So our immune system is in the gastro-intestinal tract and when it recognizes an attack from a microorganism that doesn’t play by our regular gi gang of microorganisms, it will set the alarm and create inflammation, increase temp, and so on. Inflammation creates pressure. Pain that arises from pressure doesnt respond to pain medicine —because its pressure. Thats why its soooo painful. Inflammation also makes tissue have more water and ergo the watery stool.