Viosterol is an older name for vitamin D2 — ergocalciferol, the plant- and fungus-derived form of vitamin D. The name dates from the 1920s, when viosterol was the commercial preparation of irradiated ergosterol used to prevent rickets, before the vitamin D family was fully understood. Vitamin D2 is made by ultraviolet irradiation of ergosterol in yeast and fungi, and it raises blood vitamin D levels much like vitamin D3 from animal sources and sunlight, though D3 is now generally preferred for supplementation. The word survives in historical and pharmaceutical contexts. Its story is part of the larger triumph over rickets — a deficiency disease conquered by understanding a vitamin.