Death: The end of any living being. Your cells age over time and reach a point where they can't reproduce as fast as their mortality rate. When that ratio gets to a certain point, your body fails to preserve itself and thats when you die. The cause of death can be variable, as we have many diseases, some which are cured (by science) and some we haven't found a cure for.

Now, a disease, is when other micro or seen one or multi-cell organisms infest another living body's cells and cause the destruction of that cell complex by either producing a chemical that reacts with the cell causing in its death or feeding on the cells themselves, not forgetting the fact that some of these organisms manipulate cells in a way that they don't serve purpose for the human body (Aka Cancer).

-ShazaM