The original worker's compensation concept envisioned compensation for workers who where injured in on-the-job accidents.
What constituted an accident varied form state to state.
However, all original definitions had the common the characteristics of being sudden and unexpected.
Over the years, the definition of an accident has undergone continual change.
The major change has been a tred toward the elimination of the sudden characteristic.
In many states, the gradual onset of a disease as a result of prolonged exposure to harmful substances or a harmful environment can now be considered an accident for workers' compensation purposes.