Burden of Proof. Adam v. Ledesma-Cadhit
In Adam v. Ledesma-Cadhit (Ont CA, 2021) the Court of Appeal considered causation in a medical product liability case:
(2) Causation. Snell v Farrell
 To succeed in an action for negligence, a plaintiff must establish that the defendant’s breach of the standard of care caused the injury or death: Clements v. Clements, 2012 SCC 32,  2 S.C.R. 181, at para. 6. In Rothwell v. Raes (1990), 1990 CanLII 6610 (ON CA), 2 O.R. (3d) 332 (C.A.), leave to appeal refused,  S.C.C.A. No. 58, it was alleged that the administration of a vaccine to an infant had caused him brain damage. This court stated, at p. 333, that unless it was established that the vaccine could cause such damage, the plaintiff could not succeed. If such a general causal relationship was found to exist, the question became whether the vaccine did cause the damage suffered by the infant plaintiff.
In Snell v Farrell (SCC, 1990), a leading case on the issue of causation in tort, the Supreme Court of Canada confirmed that the ultimate burden of proof lies with the plaintiff, but in terms of the evidentiary burden drew up the evidentiary principle of 'adverse inference', rather than any concepts of (1) prima facie proof resulting in a shifting of burden to the defendant, or (2) legal v evidential burden. The court stated as follows:
These references speak of the shifting of the secondary or evidential burden of proof or the burden of adducing evidence. I find it preferable to explain the process without using the term secondary or evidential burden. It is not strictly accurate to speak of the burden shifting to the defendant when what is meant is that evidence adduced by the plaintiff may result in an inference being drawn adverse to the defendant. Whether an inference is or is not drawn is a matter of weighing evidence. The defendant runs the risk of an adverse inference in the absence of evidence to the contrary. This is sometimes referred to as imposing on the defendant a provisional or tactical burden. See Cross, op. cit., at p. 129. In my opinion, this is not a true burden of proof, and use of an additional label to describe what is an ordinary step in the fact-finding process is unwarranted.
The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield's famous precept. This is, I believe, what Lord Bridge had in mind in Wilsher when he referred to a "robust and pragmatic approach to the ... facts" (p. 569).