what is the reason people die naturally even though they have no known disease?

I see people become physically weak at times they get old and die eventually in case they are not attacked by any known disease.

My question is what reasons leads human to death?

  • $\begingroup$ What Does it Mean to "Die of Old Age" $\endgroup$ – aaaaa says reinstate Monica Aug 1 '15 at 0:19
  • $\begingroup$ the question, called senescence is kind of an open one in evolutionary biology. There are some organisms which appear not to die - dividing amoeba and perhaps the microorganisms called hydrae. $\endgroup$ – shigeta Aug 1 '15 at 2:09
  • $\begingroup$ Because people aren't supposed to live forever. We evolve trough generations. $\endgroup$ – Trevi Awater Aug 1 '15 at 8:23