In Chapter 19 of "Tale of Two Cities", what is the only thing the Doctor says could bring about another relapse?

The Doctor says that the only thing that could bring another relapse to the patient is "a strong and extraordinary revival of the train of thought and remembrance that was the first cause of the malady". In other words, there has "long been a dread lurking in his mind", but the patient, being well now, is no longer aware of it - as long as he does not remember it, he is fine.  The only thing that would cause another relapse would be if something should happen to jar his memory of the events which so traumatized him in the first place.  The Doctor does not think it is likely that the patient will suffer another relapse, because the "circumstances likely to renew it are exhausted", and will not happen again.

There is a certain pathos in the Doctor's evaluation, because the patient he is diagnosing is himself.  With cautious tentativeness, he expresses his feeling that "the worst is over", never realizing that in reality, the worst is yet to come (Book the Second, Chapter 19).