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).
Charles Darnay and Lucie are now married and they depart on their honeymmon. The trauma of being separated from his daughter is too much and suddenly without himself being aware of it, Dr.Manette relapses into being a cobbler. This strange condition lasts for nine days. However on the tenth morning Dr.Manette is his usual self. Mr Jarvis Lorry wishes to avoid another relapse. But he has to do it without Dr.Manette himself being alerted to his psychological condition. So very ironically, Mr. Jarvis Lorry discusses Dr.Manette's psychological disorder with Dr.Manette himself but in an indirect manner: Mr.Jarvis Lorry tells Dr.Manette that an imaginary friend of his is suffering from the same symptoms as Dr.Manette and seeks his advice on how to treat him.
Since there has been a relapse in Dr.Manette's own psychological condition, Mr. Jarrvis lorry wishes to know whether there will be a relapse again and how to prevent such relapses in the future. To which the doctor replies, that if the same associations and train of thoughts which brought about the first instance of nervous disorder were recalled and the same stimulus applied then there would be a relapse:"That there had been a strong and extraordinary revival of the train of thought and remembrance that was the first cause of the malady. Some intense associations of a most distressing nature were vividly recalled."