A similar formation appears in Old English wes þu hal, but this is not recorded as a drinking salutation. Sense extended c. 1300 to "liquor in which healths were drunk," especially spiced ale used in Christmas Eve celebrations. Meaning "a carousal, reveling" first attested c. 1600. Wassailing "custom of going caroling house to house at Christmas time" is recorded from 1742.
"of sound mind, mentally sound; free from disorder," 1721, a back-formation from sanity or else from Latin sanus "sound, healthy," in figurative or transferred use, "of sound mind, rational, sane," also, of style, "correct;" of uncertain origin. Perhaps from PIE *seh-no- from *seh- "to tie." That reconstruction "is purely mechanical," according to de Vaan, the meaning might be "which is in place, in order." Or it could be from a different root meaning "to satisfy" as in Latin satis "enough." Used earlier, of the body, with the sense of "healthy" (1620s). Related: Sanely.
1914, from German Gesundheit, literally "health!", from Old High German gisunt, gisunti "healthy" (see sound (adj.)). Also in the German toast auf ihre Gesundheit "to your health." God bless you after someone sneezes is credited to St. Gregory the Great, but the pagan Romans (Absit omen) and Greeks had similar customs.
also auto-immune, "arising from an abnormal immune response to a normal body part," 1952, from auto- + immune. Related: Autoimmunity, attested by 1903 as "immunity, natural or acquired, effected by the unaided powers of the organism, independent of external agencies." Modern sense of "immune responses of an organism against its own healthy cells and tissues" is from 1950s.
1847, in reference to health; 1864 in reference to salvation, from German soteriologie, from Greek soteria "preservation, salvation," from soizein "save, preserve," related to sōs "safe, healthy," which is of uncertain origin (perhaps from PIE root *teue- "to swell," on the notion of "to be strong"). With -ology.