We'll use a golf course for this example. Your golf course has just been fertilized, wall to wall. Much of that dissolved fertilizer runs off of the course with the water to the ditches, streams and rivers. That nutrient rich water, combined with other nutrient rich water from other golf courses, farms, feed lots, businesses and homes, creates a nutrient rich environment in the water body which provides food for bacteria and fungi to grow. As they reach higher populations, out of balance, they infect other wildlife dependent on that water.
Fish become deformed or die, animals which feed on the fish become sick too.
Here in Florida on golf courses, two weeks after a fertilizer application, the algae explodes in the lakes on the golf courses. They then treat the lakes to control the algae. Much of agriculture and medicine practiced in this country has focused on treating the symptoms of a problem, not the cause. Not what can we do to prevent you from getting cancer. It's the same in agriculture.