Chazal said about the verse “And he will surely heal!” that we see from here that the Torah gives permission to the healer to heal. They said further about the verse “And bad faithful illnesses” that the angels are faithful in their jobs as when they send illnesses to people they decree that they should only disappear on a certain day by the hand of a certain person by using a certain medicine. Chazal also warned a person against living in a place where there is no doctor.
Regarding this idea each person can ask, “Being that we know that Hashem gives and removes life, what can the doctor give or add? Surely if it is decreed that a person should die even if all of the doctors in the world would stand there they would be unable to save him from death? And if, on the other hand, it has been decreed that he should live, Hashem can bring about his cure even without the help of the doctor as Hashem has many ways of achieving things and He will send the cure and heal him?”
The answer to this (and to many other similar questions) is that there are three types of decrees: 1) There is a person who has many merits and has a strong destiny and even if he does not go to the doctor and he does things which make the illness worse, he will nevertheless not die but will live. 2) There is a person whose death has been decreed in heaven (G-d forbid) in which case even if he guards himself with all of the different possible precautions even the doctors will be unable to help him and he will be doomed. 3) There are people who because of their sins are left to the power of nature and if they will guard themselves and use the powers of cures which work according to nature will live, whereas if he does not he will die. About this type of person the verse says “Some are gathered without justice!”.
It follows that every person should worry that maybe he is a member of that group pf people who are given over to the laws of nature and he should therefore occupy himself with finding cures when he is not well, as, if he does not do so and dies he has sinned with his soul as he has transgressed the biblical obligation of “And you shall guard your souls very well!”. A person who is not careful and does not conduct himself according to the laws of nature to seek a cure for his illness, apart from the fact that he will surely be punished for having transgressed that which is explicitly written in the Torah, there is also another problem in that he is relying on a miracle and if he receives a miracle to save him he will have the merits for that miracle deducted from his merits.