Epidemiologic studies provide quantitative information about the pathologic role of a single risk factor in large populations, but available biostatistical data are not sufficient to apportion liability when exposure to more than one potential risk factor has occurred. Given this scientific void, some courts, upon a threshold demonstration of negligence, have shifted the burden of proof regarding causation to the defendant--forcing him to prove a negative--that he did not cause the plaintiff's injuries. To the extent biologic markers become a scientifically acceptable and legally reliable means of proving that exposure to a particular risk factor caused a specific disease, judicial decisions regarding disease causation can be made with scientific certainty and without subjunctive reference to the defendant's purported negligence.