Lymphocystis is akin to the common cold, it is ubiquitous (it is present everywhere in the aquatic environment). Just as most colds are self limiting, any cold can develop serious complications, and as you know, Lymph can become lethal to fish under stress.
There is a certain amount of immunity fish can build up if they beat it, but can also become non-symptomatic carriers.
I have had fish develop Lymph due to the stress after being shipped, yet with care go into remission for the rest of there lives.
I would never sell or give away a fish with symptoms, but unless you have an entirely sterile environment, where fish are, and have always been totally isolated from all other fish, and any outside influences, Lymph, though asymptomatic, is always there.
You must decide if the strain in your tank is so robust or epidemic, that it warrants euthanizing the fish, or controllable with TLC.