We tend to think of the human body as a single living thing, but it isn't.  It's really a colony of many living things that have to work together to keep the whole living and functioning.  For example, we have bacteria living in our digestive tract and mouth that are essential.  Without the bacteria, we could not digest food and we would literally starve in the midst of plenty.  Yet, we seem determined with our army of anti-bacterial products, to destroy all bacteria, the good with the bad.  
    We used to think that all decision making had to be done in the brain.  Now it is known that some simple decisions that have to be made quickly, such as pulling your hand away from fire, are made in the spine before the knowledge that you are getting burned even reaches the brain.  And other major nerve centers such as the solar plexus are making simple decisions also.
    Now try to imagine what would happen if all these various parts of you decided it was every man/women for himself/herself.  Like if your nose decided it would stop you from breathing so it could keep out the pollen that irritates it.  Good for the nose maybe, at least for a while, but not good for the whole.  And what if your heart decided it needed a day off?  Not good for you either.  
    Occasionally some part of you does decide to do it's own thing, and ignore what is best for the whole body.  This is called cancer.
And when enough of you gets cancer, you die.  
    This seems obvious to many, how could it be any other way?  All parts of the body must work together for the benefit of the whole.  Isn't that every bit as true for the human race as a whole?  That, we often forget.