Koi fish today are not the same tough fish as their ancestor, the magoi carp. The magoi carp are extremely resilient fish. They can survive in very poor water conditions and host a serious parasite population without getting sick. Because our modern day koi have been selectively inbreed, so much, to produce the beautiful colors and shape that we all love, they have lost much of their ability to endure such harsh treatment. Still, today's koi seldom get sick and or die without a reason.

Of the top 4 problems in our ponds poor water quality takes the top of the list and invites a host of bacteria's, and protozoans to enter our ponds therefore making our fish ill.
Parasites can drop in or be carried in by pond visitors such as frog, birds etc.
Ulcer disease is common with Aeromonas bacteria which naturally occurs in pond water.
Predators will always be drawn to our ponds sooner or later.
Of the last 6 problems of why Koi die are really more common sense things. Be sure and aerate your pond. Don't over medicate and use proper doses. Keep your pond netted. Prevent virus's by knowing who you are purchasing your fish from. And make provisions so that water that has chemicals from your lawn does not run into your pond.

Common sense keeps your koi healthy..........