There is a legal concept called the "eggshell skull doctrine". The idea is that there was a person who had a genetic condition in which their skull was as thin and brittle as an eggshell. People horsing around carelessly pushed someone into the person, fracturing their skull, causing immediate hospitalization and lifelong follow up medical attention. But for the carelessness and negligence of the bad actors, this person would have gone along without problem. So, yes the negligent actor or actors must pay. This is where an old saying comes from that the defendants in a lawsuit or the negligent parties must "take the plaintiff as they find them". To decide whether negligent conduct was more than an insignificant factor in bringing about harm, the conduct can be found to be a contributing factor if the action alleged to have caused the harm was an actual, real factor, not a "negligible, imaginary, or fanciful factor, or a factor having no connection or only an insignificant connection with the injury." However, factual cause does not mean it is the only, primary, or even the most important factor in causing the injury. A cause may be found to be a factual cause as long as it contributes to the injury in a way that is not minimal or insignificant. What all of this language means is that where a person, by way of negligence, causes an accident and a resulting injury, they are legally responsible for the aggravation of any preexisting harm. A person may have been in a prior accident, injuring a body part. After treatment the body part is, say, 90% better. There may even be a "predisposition" to later problems or issues with that body part. Along comes accident # 2 and another injury to the same body part. Even if the injury seems excessive, the negligent party is now liable to the injured person for the aggravation of the injury. The person was humming along in life symptom free, and as long as medical evidence shows the extent of damage caused by injury # 2, actor # 2 is responsible for all of it, unless their own medical evidence can distinguish by a preponderance of the evidence what symptoms are an aggravation, and what symptoms would have been there anyhow as a result of accident # 1. This is the law, as summarized in this fairly typical jury instruction: "Damages should be awarded for all injuries caused by the accident even if the injuries caused by the accident were more severe than could have been foreseen because of plaintiff's prior physical condition or a preexisting medical condition was aggravated by the accident. If it is found that the plaintiff did have a preexisting condition that was aggravated by the defendant's negligence, then the defendant is responsible for any aggravation caused by the accident."