You have situations like the following:

- The age-adjusted baseline rate of a particular type of cancer is, say, 1%.

- An age-adjusted 2.5% of people in an afflicted community have been diagnosed with this type of cancer. This is a huge increase above baseline.

- By implication, of people in the community with a diagnosis of this type of cancer, 40% of them would have cancer anyway for reasons unrelated to whatever is being sued over, and 60% of them got it "because" of whatever the problem is.

It isn't possible to make any distinction between those groups at the level of an individual case. There's no difference in the cancer.

McGhee v National Coal Board is probably the most instructive case on the matter. It brought a 'material increase in risk' test to apply to the causation step of analysis. The case has been widely cited and has had international impact.

https://en.wikipedia.org/wiki/McGhee_v_National_Coal_Board

Your jurisdiction might deal with things differently, but this case has had international adoption, as the framework deals with scientific uncertainty fairly elegantly.