Toilets in the developing world are most commonly located over pits. Septic trucks are hired to pump out the sewage and carry it away. Too often, the dumping site for the sewage is a river, stream, bay, ocean, etc., with no further treatment whatsoever.
This creates a huge problem. The sewage contaminates the drinking water, causing people to get sick.
When pit latrines fill up, people have no choice but to defecate in the open. This ends up running into open drinking water.