Why is it that people die?

[Master_of_the_Faster]

Why do people die in the physical sense (not as in the sense where someone would die because you forgot that they existed or someone else is dead to you because you have no emotion towards them now)? I understand that as people age, the strength of their bodies' immune system declines, but why? Is it because of the fact that our immune system is constantly at battle against other bacteria or what? Does anyone believe that it is a possibility that a person could find a way to live for ever based on the technology we have today?