Unfortunately, due to numerous diseases, and numerous cases of these diseases, modern Jewish literature discusses virtually every scenario and situation imaginable. Often the individual case involves very specific features and details that are particular to that case, and therefore it is always wise and correct to consult with a qualified Rabbinical authority for guidance in how to deal with each individual case.
Having said that, I am aware of a clear ruling that answers your specific question. The basic dilemma here is whether to stick to the truth and say that you don’t know if the illness will pass, or whether to “exaggerate” and act more optimistically towards the patient so that the patient will “feel good”. This question was, in fact, posed to a great Halachic (Jewish Law) authority in Israel — Rabbi Yitzchak Zilberstein, the Rabbi of the Ramat Elchanan community in Bnei Brak. I will try to summarize his response, briefly yet accurately.
The correct thing to do in this case, said the Rabbi, is to deviate from the truth. Despite your own personal professional lack of certainty regarding the outcome, it is proper in such a case to assure the patient that from a medical point of view the illness will pass. He cites a precedent basis in Judaism for being optimistic in this case. Jewish Law permits a eulogizer to exaggerate the praise of the deceased in a eulogy that he delivers. This is permitted in order to honor the deceased. Therefore, said the Rabbi, it is certainly proper to exaggerate in order to spare a living person from anxiety.
Rabbi Zilberstein added an anecdote about a follower of a Chassidic leader whose son was mortally ill. When the Chassidic leader promised him that his son would live, the many people who heard this promise wondered how he could go out on a limb with such a promise that was “against the odds”. The Chassidic leader’s reply was that all he could fear if he was proven wrong was that his followers would lose faith in him and abandon him.
But that didn’t worry him. This righteous man explained: “All of that doesn’t matter if I can bring a little comfort to a sick person and his family.”
