God’s Permission To Heal
At the core of Jewish belief is the profound understanding that God controls everything that happens in this world. “No man bangs his finger below (in this world) unless it was announced above (in Heaven)” (Chulin 7b). Therefore, if God decrees one to be sick, God also decrees one to be cured. Using this logic, it would seem that medical professionals should cease to exist because it is up to God for a person to be healed. Whatever a person is destined for, that person shall receive, no matter the medical intervention given.
There is a machlokes brought down in the Gemara whether one says a specific prayer when undergoing bloodletting. Abaye says that one does not because of the pasuk of Vrapo Yerapei (Shemos 21:19), and therefore, God permitted man to heal. This suggests that a doctor would not have the authority to heal logically, but the Torah bestows special permission. However, this raises the question of why a physician is permitted to 'oppose' illness and suffering and how their efforts can be effective if illness and suffering are considered Hashem's decree.
Chazal gives a beautiful explanation (Midrash Temura 2):
R’ Yishmael and R’ Akiva were once walking in the courtyards of Jerusalem accompanied by another man. They met a sick person who said to them, “Rabbis, tell me how I will be healed?” They replied, “Do such and such until you recover.” He asked, “And who struck me (with illness)?” They answered, “haKadosh Baruch Hu.” He replied, “[If so] you are intervening in a realm that is not your own. He struck [me with illness], and you are healing me! Are you not countering His will?” They asked, “And what work do you do?” He answered, “I work the land; indeed, here is a sickle in my hand.” They asked, “And who created the vineyard?” He answered, “haKadosh Baruch Hu.” They exclaimed, “You are intervening in a realm that is not your own. He created it, and you are cutting it down!” He replied, “Do you not see that sickle in my hand? If I wouldn’t have plowed the land, weeded it, fertilized it, and hoed it, it wouldn’t have produced anything!” They answered, “Foolish one! From your work, do you not hear what is stated: “Man, his days are like those of grass.” Just as a tree will not grow if one doesn’t hoe and fertilize and plow, and if it grows but isn’t watered or fertilized, it will not live but will die, so too is the human body – the fertilizer is the [equivalent of the] medicines or the types of Refua and the man who works the land is [equivalent to] the doctor.
Obviously, illness comes from God, but it is also part of nature. Just as a farmer tends his field to produce fruit, he must also protect his body from harm and obtain medical treatment in case of illness. In short, a medical professional does not work against God's will; he is considered God’s emissary when saving the lives of his patients.
Adapted from medicalhalacha.org