Koi are no different to humans, when it comes to illnesses and diseases, they are also susceptible to all the nasties out there, just like us, Koi are extremely tough and hardy creatures, but they too develop illnesses if the following criteria are not upheld:

AERATION; Fish breathe oxygen in the water through their gills. If the water does not have sufficient aeration the fish will suffocate.
WATER CONDITION; Checking your ph and ammonia, nitrite levels, making sure that you have a good filtration system.
WATER CHANGES; Do small, regular water changes on a weekly basis.
CROWDING; Do not overcrowd your pond, as this places a great deal of stress on your fish as well as on your biological filter.
FEEDING; Do not overfeed and make sure that your fish are getting the right nutrition.
INJURIES; This is quite common, treat it immediately if you notice injuries.
CIRCULATION; Make sure that there is good movement in the pond, stagnant water is not good as bad bacteria grows well in stagnant water.
It is advisable to follow the above guidelines to prevent future health problems. The good old saying "prevention is better than cure" applies here.
If you take the right approach in maintaining your pond, your fish should be okay - although minor ailments do occur even under the best conditions.