Actus reus is Latin for "guilty act" and is the physical element of committing a crime. It may be accomplished by an action, by threat of action, or exceptionally, by an omission to act, which is a legal duty to act. For example, the act of A striking B might suffice, or a parent's failure to give food to a young child also may provide the actus reus for a crime.
Where the actus reus is a failure to act, there must be a duty of care. A duty can arise through contract, a voluntary undertaking, a blood relation with whom one lives, and occasionally through one's official position. Duty also can arise from one's own creation of a dangerous situation. On the other hand, it was held in the U.K. that switching off the life support of someone in a persistent vegetative state is an omission to act and not criminal. Since discontinuation of power is not a voluntary act, not grossly negligent, and is in the patient's best interests, no crime takes place. In this case it was held that since a PVS patient could not give or withhold consent to medical treatment, it was for the doctors to decide whether treatment was in the patient's best interest. It was reasonable for them to conclude that treatment was not in the patient's best interest, and should therefore be stopped, when there was no prospect of improvement. It was never lawful to take active steps to cause or accelerate death, although in certain circumstances it was lawful to withhold life sustaining treatment, including feeding, without which the patient would die.
An actus reus may be nullified by an absence of causation. For example, a crime involves harm to a person, the person's action must be the but for cause and proximate cause of the harm. If more than one cause exists (e.g. harm comes at the hands of more than one culprit) the act must have "more than a slight or trifling link" to the harm.
Causation is not broken simply because a victim is particularly vulnerable. This is...