Musar zu Mischlej 13:23

And thus we find that an [interesting] inquiry arises. Since it is Hashem who determines death and life - what what will the doctor add [by aiding to heal the patient]? Is it not so that if it was decreed [in Heaven] that one will die, even if all the doctors in the world arose to heal him, they would not be able to save him from death?And thus [we can better understand] our sages statement "the error / malpractice of the doctor is the will of the Creator" and it is not in his power to save. And if it was decreed upon the person that he should live, even without a doctor, the Holy One Blessed Be He, will turn over [worlds] and cure him. For Hashem has many ways and with Him there is much deliverance and He will stretch out his word [other version "hand"] and heal him. However, the answer to this inquiry and to many others like it is that there are three different types of decrees - 1) If he is one that holds a special merit and his merit is strong and his fate is salutary - even if he doesn't have a doctor and he does things that will worsen his condition, he will not die and "live he will live". 2) And there are those, upon whom death is decreed - that is to say, even if he guarded himself with all protection in the world, his persistence of protecting himself is all in vain, and doctors will be of no help and he will not be saved. 3) However, there are those who, due to their sins, place themselves under the influence of "nature": - if they protect themselves and act with the proper cures according to nature - "live he will live" - he will surely live. - and if not they will die and on this type of person it is said "and there is death without judgment."