I'm not sure why this seems so difficult for some.  We have a flesh body which is corruptible....we have a spirit which is not.  We have a gatekeeper called the Holy Spirit which allows or disallows depending on certain factors.  Our armor needs to be in place at all times....or we allow a portal for darkness.  

How can we say that evil can not corrupt our bodies?  If God can coexist with satan in heaven....how can we not believe that the same can be true in our temple?  Only when satan was cast out of heaven did the courts of heaven stop hearing his case against us.  The demons need permission (legal right) to harm us.  We give them legal right, when we break God's laws.  The idea is to do what is right and keep them out by being covered in our armor and filled with the Spirit.  The less filled we are, due to more and more unrepentant sin, the more cracks in that armor we have.