Adam and Eve rebelled against God taking on a consciousness separate from God and they were put out of the Garden of Eden and were denied access to the tree of life. Their minds were veiled. Their new minds could not contemplate immortality and this determines how their bodies worked and they then aged.
We are then, all brought up to believe a certain way and our character and personality develop, revolving around our own self. We are unable to even imagine the mind required to be immortal.
The veil in the mind to knowing God is all those things that go on in the mind. Promptings, temptation, imaginations and these can become actions and behaviors. The result is that there is something standing in the place of God in the mind that tends to dominate and rule the personality and this is Satan. Therefore all usual human endeavor and behavior and interaction and the presence of a person has a ruler and this represents his kingdom, the kingdom of Satan. This kingdom brings all the emotional response, impulsiveness, diseases, sickness and deformities, even mutations and the predatory biology, the decay and oxidization and entropy of this planet and universe. I know this sounds impossible but we have our being and presence in God and God is clothed in light and with a sweep of His hand everything is brought into existence. God manifests everything instantly and all that is seen is made to appear as we see it all the time.
But when Adam and Eve rebelled all creation came under a subjection to a futility and they themselves and all biology and also the physical universe. Sin came into the world and this word means to miss the mark or alternatively to not measure up.
Jesus overcame sin and the human mind with Satan's control then developing a mind and nature that was acceptable to God and he was raised from the dead and is immortal and alive. Jesus is the ruler and king in the kingdom of God. If a person is willing they can become recipients and participators in what can start off as something of an act but should go deeper and become real as the mind changes. The Kingdom of God will bring a new heaven and earth and Satan will be brought into total captivity and chained up. There will be no sickness or death.