So far so good. Firstly, in some cases , the reasoning for isolating the fish is questionable. If the fish is heavily infected with external micro organisms then, most probably, other fishes are infected too, or the micro organism is present in the water column. An exception is when the infected fish is a new addition which was not quarantined - the only difference being that the micro organism was not present before the infection but was introduced by you. To start with , there is nothing you can do for viral infections .
Viruses are a special king of micro organism which, simply put, is at the borderline of life.
Of course, they do not die. What they do is they transform in a new stage which is very resistant while there are no signs of life as we know it. They do not breath, they don't metabolise, move or reproduce at this stage. When you deal with a viral infection you have to rely on the good condition of the immune system of your fish. That is another good reason to keep them in prime shape and this is why stressed fish is always prone to viral infections.