"He shall pay for the loss of his work, and heal shall he heal
" (Exodus 21:19).
From here is derived that a physician is allowed to heal (and we do not say that
since G-d afflicted the person, it is forbidden to cure him).
(Talmud, Bava Kama 85a)
The young wife of Rabbi Shmuel of Lubavitch had fallen ill, and the doctors were
unanimous in their opinion that there was no hope of recovery.
When her father-in-law, Rabbi Menachem Mendel of Lubavitch, was told of the
doctors' verdict, he said: The Talmud specifically derives from the Torah
that, "A physician is allowed to heal"; but nowhere has a doctor been given the
authority to condemn a human being as incurable...