What kind of diseases can a person catch from a swimming pool that has not been properly sanitized and contains bodily fluids?

Not many, it is safe: Most swimming pools are chlorinated and the chemicals used in the water kill most of the germs. So no serious infections are transmitted from using the pool water for swimming. A quick shower(which is advised to all swimmers) after the swim removes any remaining germs that may survive inside the pool water. So it is a safe place to enjoy swimming as much as you like.