Health and Care
Koi's health is depend upon the environment provide by the keeper.  Koi have a  high resistance normally and succumb to disease usually only after exposure to stressful conditions that break down the normal immune system.  A stressed fish becomes a sick fish.  It  has been said that stress is the main factor man has the most contol over.  Many disease causing organisms normally occur in the same environment as the fish.  They usually only become a problem when present in significant quantities and or stress occurs.  Therefore, by controlling stress you can help maintian a healthy pond.  Prevention is easier and a lot cheaper  than treating your pond for disease. 
Some causes of stress are:
High ammonia level
Low dissolved oxygen level
Handling and or moving fish
Poor water quality
Overcrowding
Parasites
Too high or low water temperature
Nitrates and other toxic chemicals
Sharp edges in and around the pond
Inadequate and imporper nutrition
The best way to fight against diseases is to prevent it.  Just make sure you've got good filter and always check your water quality adn always feed your fish good food.  You can save a lot of money in prevention rather than to find a cure.
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