Pathogens (disease causing organisms) are virtually everywhere. They can't be avoided. However, most healthy fish can fight off infection because they, like humans, have a well adapted immune system. It is in the confines of a tank that this easily falls apart. Stress is the one thing you want to reduce as much as possible. The best way to do this is with clean water (0ppm ammonia and nitrite, less than 20ish ppm nitrate) of appropriate parameters (temperature, most notably, and pH and hardness to some degree). Aggression from other fish can also be stressful, as can too small a tank. Rapid fluctuations of pH and temperature (caused by playing with pH adjusting chemicals or too small a volume for stability) are also extremely stressful.
Really, if you take one thing away from this, it should be that routine water changes and a proper tank (amply large and hiding places) are the best prevention. It will prevent toxins (ammonia, nitrite, nitrate) from accumulating, it will prevent crowding, and it will allow the fish to relax. That said, I would hesitate to keep 4 Angels in a 36 gallon tank. As juveniles, it will be fine, but when they get to about 6" (and still growing), it will begin to be a problem of territory.
