Special rules relate to liability in cases of mesothelioma where the victim has been wrongfully exposed to only one source of asbestos which materially increased the victim's chances of contracting the disease. The usual requirement that a claimant must show that it is more likely than not that harm suffered was caused by the defendant’s breach does not apply because medical science cannot presently determine which asbestos fibre or fibres caused the mesothelioma to develop.