What is the test for a plaintiff to prove that negligence was not the sole cause of injury?

The plaintiff is not required to establish that the defendant’s negligence was the sole cause of the injury. The tortfeasor must take his or her victim as the tortfeasor finds the victim, and is liable even if there are other causal factors, for which the defendant is not responsible, that result in the victim’s losses being more severe than they would be for the average person. At the same time, the tortfeasor need not put the victim in a better position than they would have been in, and need not compensate the victim for the effects of a pre-existing condition that the victim would have experienced in any event: Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311; Athey.