The reason we become susceptible to disease as we age. It has been a natural way to think that people age and at the end of the very process, we face death. Who could dare to deny this fate? To most of the people, it even seems almost wise to humbly accept it as it has been thought of. However, there are researchers who study on this subject to confront death, or at least to prolong the span of our lives. It's not an easy task to find out what kinds of biological machanisms and reactions cause aging and death, but one thing we can be sure is the fact that aging-related diseases do overlap with the reasons of death, and in most cases, they lead us to death in the end. Therefore, it's rational to consider death as an irreversible conclusion or symptom of the diseases which are caused as we age, although the manner to take it is up to individuals themselves. Then, why do we get those diseases as we age? Although it might sound pretty indifferent, considering how much we care about it, one hypothesis explains the reason: we become more and more susceptible to diseases and eventually die because the evolution of humankind doesn't much care about what happens in our old ages. Taking account of the fact that reproduction takes a crucial part of evolution, and our physiological aging almost conincides with the after moment we finish our reproduction, it can be thought to imply important clues for the question.