In essence, this is the scenario of the truly vulnerable victim. This is when the effects of an accident become more difficult to predict. A mere fall could render the victim totally disabled, or some unanticipated consequences of that nature. The tortfeasor will be responsible for all the damages caused by the negligent act, even if they would not generally follow in the case of a normal person. In situations of dormant conditions, the thin skull victim is treated like any other except for the quantum of damages that takes into account the magnified effect on his or her injuries. Of course, where the effects on a victim’s life and work are more severe, the award of non-pecuniary and pecuniary compensation will generally be greater.
One of the challenges, when dealing with the thin skull rule, is accounting for active pre-existing conditions that have been caused by a prior tortious act. In other words, a victim is injured and still has sequelae when another accident overlays a new set of symptoms either identical or very similar to the existing one. To what extent, if any, can a second tortfeasor be responsible for the victim’s ailment? In Athey, supra paras 17 & 19, the unanimous bench of the Supreme Court of Canada explained that a tortious act does not need to be the sole cause of the injury in order to justify compensation:
 It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant’s negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring. … As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury.
 The law does not excuse a defendant from liability merely because other causal factors for which he is not responsible also helped produce the harm …. It is sufficient if the defendant’s negligence was a cause of the harm ….
[Underlined in the original]
In subsequent cases, some defendants tried to limit the ratio of the court pre-existing conditions related to non-tortious acts, but to no avail. Namely, in E.D.G. v. Hammer,  2 SCR 459, a case dealing with a girl sexually abused by a school janitor before being also abused by members of her extended family, the defendant argued that liability should not compensate the torts committed subsequently. Chief Justice McLachlin clarified, albeit in obiter, that the “principle is not confined to cases involving non-tortious preconditions. It applies to any case in which the injuries caused by a number of factors are indivisible” (para 31). In Blackwater v. Plint,  3 SCR 3, a child was abused at home before being also abused in a residential school. Again, the public authorities tried to split liability between the various sets of tortious acts. Regarding the pre-existing condition from the first abuses, the unanimous bench stated (paras 78-79):
 It is important to distinguish between causation as the source of the loss and the rules of damage assessment in tort. The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: Athey. …
 At the same time, the defendant takes his victim as he finds him — the thin skull rule. Here the victim suffered trauma before coming to AIRS. The question then becomes: What was the effect of the sexual assault on him, in his already damaged condition? The damages are damages caused by the sexual assaults, not the prior condition. However, it is necessary to consider the prior condition to determine what loss was caused by the assaults. Therefore, to the extent that the evidence shows that the effect of the sexual assaults would have been greater because of his pre-existing injury, that pre-existing condition can be taken into account in assessing damages.
Those principles were followed by the New Brunswick Court of Appeal in McIntyre v. Matthews, 2020 NBCA 52. The plaintiff had sustained a workplace back injury. She had been able to return to work, albeit with the assistance of medication, and although she had to find alternate employments in less physically demanding jobs. At the time of the car collision, she was working full-time in a bakery as a front-end customer service clerk, employment that she was not able to pursue afterward. At trial, and again on appeal, the tortious car driver argued that the trier of facts should distinguish between the two events and limit liability to the sole effects of the second accident. On the basis of the aforementioned quote from Athey, supra para 17, the appellate court stated that in “‘thin skull’ cases, the question of whether the pre-existing condition was dormant or active is of no moment, provided it was not degenerative” (para. 59). The two levels of court looked at the full effects of the car collision without reducing liability to account for the active pre-existing condition as the evidence showed that the workplace injury had rendered the plaintiff vulnerable. The cases of O’Neill-Jones v. Chabot, 2015 NBQB 143, Hickey v. Savoie, supra para 53, and Crawford v. Galbraith (1995), 165 NBR (2d) 339 (QB), are consistent with the approach adopted in Matthews v. McIntyre.
The cases of Chartier v. Laramée,  SCR 771, and Athey, supra, support the view that a tortious act can render vulnerable a normal victim. In Chartier, the plaintiff suffered a broken leg. After being put in a cast and sent home, he returned to the hospital for further treatments. Then, when walking with crutches while his cast was off, he lost his balance, put weight on the healing leg and broke it again. He had to undergo surgery and a bone graft. The unanimous bench attributed the reinjury to the original accident, holding that it would not have happened but for the vulnerability created by the collision (p 777):
Il est évident que si le jeune homme n’avait pas subi une première fracture par la faute de l’intimé, il n’aurait pas subi la seconde. On ne peut pas lui reprocher d’avoir marché avec des béquilles. Son médecin le lui avait prescrit. Évidemment, il lui avait également dit de ne pas faire porter son poids sur la jambe blessée mais de se supporter avec des béquilles. Cependant, sans le faire exprès, il a perdu l’équilibre en voulant passer dans une porte à ressort. Faut-il voir là une faute? Celui qui est obligé de marcher avec des béquilles n’est évidemment pas entraîné à le faire mais on ne peut sûrement pas le lui reprocher. S’il est obligé de s’y aventurer, c’est comme conséquence du premier accident dont la responsabilité est imputable à l’intimé.
Ici la preuve ne démontre pas que la seconde fracture soit le résultat d’une faute de la victime. Comme elle est évidemment par ailleurs la conséquence de la condition dans laquelle cette dernière s’est trouvée par suite du premier accident, il faut l’y rattacher.
Similarly, in Athey, the plaintiff injured his back while stretching at a gym. He had been involved in two prior motor vehicle accidents. The unanimous bench held the two tortfeasors liable for the injuries, as having contributed to the victim’s vulnerability. Technically, as far as the wrongdoers were concerned, the cases dealt with the foreseeability of the effects of an accident, not a pre-existing condition, but they still inform the relationship between an active health issue and a subsequent injury.
Thus, for a thin skull victim, the tortfeasor is liable for all the foreseeable effects of the tortious acts, even when they are greater than what a normal person would suffer. The vulnerability created by a prior tortious act is treated similarly if the sequelae from the two events are indivisible. When two accidents generate separate injuries then courts usually consider and compensate them separately.