A similar case is actually taking place now in Italy. They found a few droplets of blood from a middle-aged family man on the underwear of a 13 year old girl th

A similar case is actually taking place now in Italy. They found a few droplets of blood from a middle-aged family man on the underwear of a 13 year old girl that was killed. Even though it's blood, all this proves is that he had some kind of (inappropriate) contact with the child. On its own, it's not enough evidence to convict him of murder. Now imagine a much more explainable scenario like semen on a prostitute, and you realize how weak the evidence is if not corroborated by further evidence.