In toxic tort and pharmaceutical litigation, cause-and-effect between a chemical agent and the onset of a disease process often cannot be specifically determined. In the diet- drug litigation, for example, the experts on both sides speak of an "association" between the use of fenfluramine and the development of heart-valve damage; to say that the use of fenfluramine "causes" heart-valve damage, however, is scientifically incorrect.
How can you prove causation in such a circumstance? Answer: with expert testimony founded on epidemiological evidence.
If you are researching the standards for such proof in Illinois, a good place to start is Donaldson v. Central Illinois Public Service Company, 199 Ill. 2d 63, 767 N.E.2d 314 (2002) (pdf), a toxic tort case involving a type of cancer allegedly caused by an environmental hazard. In Donaldson, the court upheld a $3.2 million award for plaintiffs even though defendant's experts, which included the treating physicians, testified that (a) the cause of the plaintiffs' cancers was unknown, and (b) that they could not say whether the plaintiffs' cancers were caused by their exposures to the hazard. The plaintiffs, on the other hand, presented expert testimony by an epidemiologist, a toxicologist, and a medical doctor specializing in occupational medicine.
It was the plaintiffs' evidence that carried the day, according to an extensive analysis by the Donaldson court of how expert testimony can be used to establish causation in a toxic tort case.