For a coroner, any death caused by disease or old age is natural. When someone dies a violent or suspicious death, medical examiners try to determine both a "cause" and a "manner." The cause refers to the biological condition that killed the victim—in Murphy's case, sudden cardiac arrest. The manner describes all the other circumstances that led up to that particular cause. Most states recognize five different manners: homicide, suicide, accident, natural, and undetermined. If a manner of death is deemed to be "natural," then the victim is thought to have died of an internal disease process or normal deterioration of the body. Outside forces, like chemicals or human intervention, had only a minimal influence. (There are some gray areas: Death by infectious disease is typically categorized as being natural, even though the killer microbes come from outside the body.)
Billy and I got in a tiff about the semantics of natural causes. Looks like he owes me 5 bucks. :-)