Seeing that most medications used to treat patients who are depressed could make them go back into the depression state as time goes on is one fear. It might be the disease of the century but I do not think the scientific medications available have proven to cure it completely, and psychologist struggle to get patients to come back to themselves.
In all this my concern goes to how a person could become depressed just by staying consistently with depressed people? Do we say that Depression is contagious?
As you said, the problem is the "after-cure". Those people are left alone, and this is where falling again in the depression spiral may happen.
I don't know whether we say this, but I would not be surprised that the environment plays a huge role.
Cheers!