Nazi soldier with death's head collar and hat badge
The skull and crossbones was a symbol for death and, later, for indicating that a thing was poisonous. It consisted of a Human skull and two femur bones. The bones were either placed behind or below the skull.
In 1996 Los Angeles, a man wore a gray T-shirt with a skull and crossbones on it. Rain Robinson's computer also displayed a "FATAL SYSTEM ERROR" message with a skull and crossbones. (VOY: "Future's End")
On Earth, during World War II, the certain units of the Schutzstaffel, a paramilitary organization based in Nazi Germany, wore the insignia of the death's head (totenkopf), which was a stylized skull and crossbones. (VOY: "The Killing Game") In an alternate 1944, these badges were worn on the left collars of all Nazi SS soldiers fighing in that version of World War II. (ENT: "Storm Front", "Storm Front, Part II")
In 2380, when Badgey created the computer viruses to infect Jackabog's ship, each of the three colored choices was marked by the skull and crossbones. (LD: "No Small Parts")