92:1 Shabbos and (most) other mitzvos are overridden when there is a case of danger to human life. Therefore, if a person is dangerously ill - even a day-old baby - it is a mitzvah to disregard Shabbos for him. If the sick person doesn't want us to ignore Shabbos for him, we do so anyway because it would be a great sin for him to be foolishly pious and not be saved. Regarding such a person, the Torah tells us, "surely I will hold you responsible for the blood of your own lives" (Genesis 9:5).
One who hastens to disregard Shabbos for a sick person in danger is considered praiseworthy. Even if a non-Jew is present, one should try to do things through a Jew. (The Rema disagrees – OC 328:12.) Anyone who disregards Shabbos for a dangerously-ill person earns a reward even if his help was not needed. For example, if a doctor said that this person needs a dried fig and ten individuals each ran and picked him a fig, each one receives a reward from Hashem even if the sick person was treated by the first one. Similarly, in any case of danger to human life, even in a case of doubt, it is a mitzvah to disregard Shabbos for the patient and to do for him other things forbidden by the Torah because nothing is more important than saving human life. (Although, if something can be done equally effectively in a Shabbos-permissible way or a Shabbos-prohibited way, one should not violate Shabbos unnecessarily – Mishnah Brurah 328:35.) The Torah was only given for the sake of life as per Ezekiel 20:11, "which if a man do, he may live by them." We see that one should "live by" the mitzvos and not die because of them. The exceptions are idolatry, forbidden sexual relationships and murder; for these, one should let himself perish rather than violate them.
92:2 If a person says, "I know this sick person and he is dangerously ill," if there is no doctor present to tell us otherwise, he is believed and we disregard Shabbos for the patient. (This refers to a Jew, who understands the ramifications of violating Shabbos – Mishnah Brurah 328:29.) Even if he didn't say explicitly, but he said that it seems to him that we need to disregard Shabbos for the patient, we listen to him and disregard Shabbos because we are lenient in a case of doubtful danger to human life. If one doctor says the patient is in danger and requires a particular treatment but another doctor says he doesn't need it, or if the patient himself says that he doesn't need it, we listen to the doctor who requires it (even if he is not Jewish – MB 328:25). If the patient says he needs a particular treatment and the doctor says he doesn't, we listen to the patient. If the doctor says that this treatment will harm the patient, then we listen to the doctor.