The question to determine with regard to causation is summarized in Athey v. Leonati, 1996 CanLII 183 (SCC),  3 S.C.R. 458 (as modified to fit the facts of this case (at para. 41)): if it was necessary to have both the accident and the pre-existing health issues for the post-accident health issues to occur, then causation is proven, since the post-accident health issues would not have occurred but for the accident. Even if the accident played a minor role, the defendants are fully liable because the accident was a necessary contributing cause.
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