Under the thin-skull rule, a defendant must take his victim as he finds him. This includes all of the defendant’s physical and emotional characteristics. It does not matter whether the victim is aware of these characteristics.
For example, if a victim has a condition which makes them particularly vulnerable to the defendant’s actions and subsequently dies, the defendant remains liable for the victim’s death even if they weren’t aware of the victim’s vulnerability.
R v Hayward 
A defendant must take his victim as he finds him, regardless of whether an ordinary person in good health would have died as a consequence of the defendant’s actions.
R v Holland 
The thin-skull rule also applies to the victims personal characteristics, as well as their physical characteristics.
R v Blaue 
In addition any physical weaknesses, the defendant must also take the victim’s characteristics and beliefs as he finds them.