Any illness that doctors normally consider dangerous or that regular people would make haste to save a patient suffering from it is deemed dangerous halakhically, even if only a small minority of people die because of it, and therefore justifies desecrating Shabbat. Thus, one may drive a woman in labor to the hospital, even though in a clear majority of cases she can safely give birth at home (Magid Mishneh 2:11). However, one may not desecrate Shabbat on account of illnesses and risks that are generally not considered dangerous (Shevet Mi-Yehuda 1:19:2; SSK ch. 32 nn. 2 and 23).
The Sages defined certain conditions as dangerous. These include internal injuries (severe pains or wounds or internal bleeding); injuries on the back of the hand and foot (that is, infections and dangerous cuts); very high fevers; scorpion or snake bites; and eye afflictions (SA 328:3-9). The Sages determined all of these cases based on experience, and today’s doctors agree in principle, though they use different terminology to describe the conditions. This is not the place to expand on the definition of a life-threatening condition, but there is a general principle: If those present think that the ill or injured party might be in mortal danger, they immediately do whatever is necessary to help him. If they need to call a doctor, they should do so; if they need to drive him to the hospital, they should do so.
When the people nearby do not know whether the patient might be in danger, they should ask a doctor, nurse, or medic in the vicinity, or they should call a doctor. If the doctor thinks that the patient might be in mortal danger, even if the patient claims that his condition is not dangerous, they must heed the doctor (SA 328:10 and 618:1, 5).
If the patient maintains that he is in danger, then even if the doctor thinks he is not, we must desecrate Shabbat and take him to the hospital to be examined. This is because “The heart knows its own bitterness” (Mishlei 14:10), meaning that sometimes only the patient can assess his own condition. Similarly, if a sick person demands a certain medicine or treatment that, based on his experience, could save his life, we heed him (SA 618:1). We only rely on the patient’s intuition on condition that it makes some sense. However, if his illness is known and he demands a treatment that the doctors believe is ineffective, we heed the doctors (BHL 328:10, s.v. “ve-rofe”). Similarly, if the sick person is known to be hypochondriac or excessively fearful and a medically knowledgeable person is certain that he is not in any danger, we do not desecrate Shabbat on his account.
If, in an effort to be pious, one asks a rabbi whether to desecrate Shabbat in order to help someone in mortal danger, he is a killer, for while he is asking, the patient’s situation may deteriorate, and the Torah commands us: “Do not stand idly by the blood of your neighbor” (Vayikra 19:16). Furthermore, the rabbi whose students ask such questions is reprehensible, as he should have taught them that saving a life overrides Shabbat (y. Yoma 8:5; MB 328:6).