A Koi's health depends upon the environment provided by the human owner. 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.
Many disease causing organisms normally occur in the same environment as the fish.
Therefore, by controlling stress you can help maintain a healthy pond. Prevention is easier than treating your pond for disease. Vibrio One of the principal causes of fish mortality is bacterial disease. Most are gram-negative organisms. Viral I know of no effective treatment except to remove nodular growths by scraping.