If one walks around and seems to be healthy, but in fact is mildly sick or experiencing discomfort, his status is the same as that of anyone else. He must observe all the Shabbat prohibitions, including the rabbinic ones. The permission to transgress rabbinic prohibitions (as explained in the previous section) is for a regular sick person, but this permission does not extend to one who is bothered by a mild illness or ailment. Therefore, if the light is bothering him, one may not ask a non-Jew to turn it off; if he needs light, heat, or air conditioning, one may not ask a non-Jew to turn them on. Even asking a non-Jew to do these things with a shinui – which would reduce the prohibition to a shvut di-shvut – is prohibited, because all rabbinic prohibitions still apply to him (SA 328:1).
All of this applies in a case where the illness is merely irritating and uncomfortable. However, if the illness or ailment causes pain, one may relieve it by performing a shvut di-shvut. If the person suffering has a great need for light, heat, or air conditioning, one may ask a non-Jew to turn them on using a shinui, such as with his arm. However, a Jew may not do so even with a shinui, because there would still be a standard rabbinic prohibition (shvut), which applies even when there is some pain (SA 307:5; 328:25; above 9:11-12; Harĥavot).
If one’s fingernail has been torn off most of the way and is bothersome, this is considered an ailment, and one may not remove the nail even via a shvut di-shvut. However, if the torn nail is painful, one may remove it using a shinui, such as with one’s hand or teeth. Since most of the nail has already been torn off, it is viewed as if it has already fallen off, so the prohibition to tear it off completely is only rabbinic. For this reason, the Sages permitted removing a nail with a shinui if it is causing pain (Shabbat 94b; SA 328:31; above, 14:2). If the nail was not torn off most of the way but is painful, one may ask a non-Jew to remove it with a shinui, since doing so reduces the prohibition to a shvut di-shvut.
Similarly, if a splinter becomes embedded in a person’s skin and it is clear that removing it will cause bleeding, one may not remove it if it is merely irritating. However, if it is painful, one may remove it, since the prohibition of causing bleeding in this way is only a shvut di-shvut, as one does not intend to cause bleeding, and the bleeding is effected via a shinui, as a side effect of removing the thorn (see MB 328:88; above, ch. 9 n. 3 and 14:2).