The general test for proximate cause is whether or not it was reasonably foreseeable that the defendant’s conduct would injure the plaintiff. Wagon Mound No. 1.
An injury doesn't have to be likely or probable in order to be foreseeable.
The risk to be reasonably perceived defines the duty to be obeyed.
Another rule used sometimes in determining the issue of proximate cause is the direct cause rule. Polemis. The direct cause rule provides that a defendant is responsible for all consequences which follow in unbroken sequence, without an intervening efficient cause, from the original negligent act. Such consequences are natural and proximate, and the defendant’s negligence is the proximate cause. The defendant does not have to foresee the exact kind of harm as long as some harm was foreseeable.
The rule from Palsgraf is often used when addressing proximate cause issues when the plaintiff is not directly involved in the event or accident that leads to his injury. Palsgraf provides that proximate cause extends liability to those whose conduct harms persons within the zone of the reasonably foreseeable plaintiff. The concept of foreseeability limits liability to the consequences of an act that can reasonably be foreseen. If a person is not a reasonably foreseeable plaintiff, a defendant cannot reasonably foresee an unreasonable risk of harm to him; therefore, the defendant’s negligence would not be the proximate cause of an unforeseeable plaintiff’s injury.
A defendant is responsible for harms that have only a small risk if the resulting damage is great. A defendant is liable for a risk that does not have a disadvantage to preventing it, no matter how small the risk is. A defendant may be liable for even a small risk if a great harm would result. Wagon Mound No. 2.
The "eggshell skull" rule states that one is liable for all physical injuries even if another person wouldn't have suffered those injuries. Simply put, a defendant takes the plaintiff as he find him.
When a third person intervenes between the defendant's conduct and the plaintiff's injury, liability turns upon whether the intervening act is a normal or foreseeable consequence of the situation created by the defendant's negligence.
A superseding act breaks the causal nexus of proximate cause. An intervening act may well be superseding if it is extraordinary under the circumstances, not foreseeable in the normal course of events, or independent of or far removed from the defendant's conduct.