This seemingly innocuous question is, from a scientific standpoint, fairly sophisticated.
At the base of the question is, “What changes to an aging living organism cause the cessation of life functions?” Comparing by analogy the living organism to a machine, the answer lies in the necessary conditions for life to exist: Fuel, Air (Oxygen), and Energy. In the case of humans, that can be stated as Food, Oxygen, and Combustion/Digestion. Add to these the proper conditions of heat, moisture, and connectivity of parts (organs, nerves, circulation, etc.), and we see that Aging is the process of exhausting these essential elements, of wearing out the “machinery.” The lungs lose their flexibility, the blood vessels harden and clog, digestive organs lose their capacity to turn “fuel” into nourishment. So aging is the wearing-out of these body machines – heart, lungs, stomach, etc. – to the point where they no longer function. Disease is the invasion of the body’s “system” from outside. One of the worn-out pieces of machinery is the body’s self-repairing mechanisms, the immune system, etc. One could ask, “What is the relationship of not maintaining or replacing factory machinery to a factory’s closing down?”