The work does not have to be the only cause. It is enough if the work causes, contributes to, or aggravates a condition which results in disability. Some of us can lift 200 pounds without any difficulty. Some of us, however, would severely hurt our back if we lifted 100 pounds. The law does not make this distinction. If a person does something at work that causes him or her to become disabled, the worker is entitled to benefits. It does not matter if there was some pre-existing weakness or if the worker was born with some condition that made him or her more susceptible to injury. This is an old principle of law that has been applied by the courts to all kinds of damage actions, including workers' compensation.

There are some special rules for certain conditions. In cases of heart disease, mental disabilities, and conditions of the aging process, the worker must prove that the employment aggravated or accelerated the condition in a significant manner. In cases of mental disability, the condition must be caused by actual events of employment. A worker is not entitled to benefits if he or she simply imagined something at work which caused the disability.