For whatever reason, medications will kill them.  They seem to be prone to ulcers more than anything else, and salt is the only treatment.
Fish symptoms and treatments are as follows:

• Fungus   A fish will have a cottony appearance,  look like it is sprinkled with salt, or have fins rotting away.  (I've seen very serious fungal infections where the fish looks like it's growing a small forest of greenish algae on its body.  This advanced fungus has a poor prognosis).  All of these symptoms are treated with Paracide Green.  Three to four doses are needed at 3 to 4 day intervals.  Do not use this medication with carbon filters.  Paracide Green will knock out the bacteria in biological filters, so we suggest re-seeding the bio filter with Bio Clarifier or Nitrifier after treatment.  This med is also an effective treatment against fish parasites, although Trichloricide is a better choice.
  
• Parasites   Symptoms are worm-like projections hanging on the gills or fish's body.  Fish lice look like a dark flea imbedded in the side of the fish.  With parasites, the fish may "flash" by rubbing itself on objects to attempt to dislodge the parasite.  Treat with Trichloricide once a week for four weeks.  After treatment, it is best to do a 50% water change and replace the filter media.  Remove snails before treatment!
  
• Ulcers   Tough to treat.  Occurs with weather changes, worse in early spring and the heat of summer.  There will be open sores anywhere on the body of the fish.  This is quite contagious, progressive, and fatal if not treated.  We suggest treating aggressively with salt and daily doses of Oxollium.  Oxollium will interfere with biological filtration, so re-seed after treatment.  Oxollium is not an inexpensive product, and with daily treatment it is best to use an isolated hospital tank with strong aeration.  It may take awhile to see improvement.
  

• Red Spot Disease or Bloody Streaking in the Fins   Both signs are indicative of a systemic bacterial infection.  With Red Spot, the underside of the fish looks like it's sprinkled with paprika. Treat daily with Oxollium.
  
• Bacterial Gill Disease   Gill covers hang open.  Treat with BGDX.  This medication also prevents disease from overcrowding, stress, and poor water quality.
  
• Dropsy   This fish ailment does not have a successful treatment regimen that I am aware of.  It is uncertain if it is contagious, but I would treat it as if it is.  The fish looks bloated with scales puffed out like a pine-cone with eyes protruding from the head.  It is best to euthanize the fish by putting it in a bag of water in the freezer. 
  
• Carp Pox   This usually occurs in cold water during the  spring.  Symptoms are waxy-like bumps on the body of the fish, often where the fins meet the body.  This is thought to be a viral infection with no known real treatment.  Often symtoms disappear as the water warms up.
  

Prevention, Symptoms and Treatments

       There are numerous factors that can cause your fish to get sick.  Factors beyond our control are weather, temperature fluctuations, and the change of seasons.  Most factors we can influence though to create a healthy environment for fish, thus preventing problems.  These are abundant aeration, clean water quality, keeping a hole open in the ice during winter, and avoiding an over-population of fish.  It also helps to feed  with fish food specific to outdoor water garden conditions. 
In general, all fish diseases are quite contagious to other fish in your pond.  And left untreated, they are fatal.  The good news is that almost all maladies are treatable.  It is important to catch problems early and treat quickly. 
A vital initial consideration is choosing a quality source to buy your fish.  At Shady Lakes, we are quite picky about who we get our fish from.  Even if the fish may cost us more from a healthy source, it is worth it to avoid importing problems.  When we bring fish in, we isolate them for one to two weeks before offering them for sale.  We avoid crowding them and provide strong, constant aeration.  Any fish with symptoms of disease are isolated and treated aggressively.  Also, we offer a one-week guarantee on our fish.  (Most fish vendors do not offer a guarantee.  You take them home, and you are on your own.  But I feel our customers should have more back-up than that.)  We do a prophylactic treatment with a broad-spectrum medication and add salt to the tanks. 
Salt?  We routinely treat all our goldfish and Koi tanks with salt that we buy from feed stores.  The first time I heard this I was startled.  Yet I have seen salt treatments bring fish back from the edge of death.  Salt invigorates fish, heals them of traumas, and moderates water chemistry.  The routine dose is one pound of salt per 100 gallons of water.  If fish are terribly sick, we will triple that (in a tank with no plants!) 
In general, the average pond owner does not need to salt their fish as medication is usually quite adequate.  To salt fish, it is best to have them in an isolation tank as salt will damage plants.  (If you must treat with salt in a pond with plants, start with a light one pound of salt per 100 gallons of water and monitor how the plants respond before increasing the dose).  All medications that we carry are safe for plants. 
We evaluate all incoming fish for any signs of problems, because even healthy fish from an excellent source can become sick simply from being stressed by being moved.   The bacterias, funguses, and parasites that cause fish problems are always present in the pond environment.  It's when the fish are compromised by over- population, moving and handling, poor oxygenation, or poor water quality that they become vulnerable to illness.  It's just like us humans catching the cold or flu when we get run down by fatigue, stress, lack of rest, or poor nutrition.

One very important point to remember is that you cannot treat golden orfes with medication!