Demonic possession occurs when an evil spirit hijacks the biological system of an organism, corrupting it and taking control of its functions. Although powerful, the possession eventually breaks down the body, degrading it as time goes on, due to its incompatibility with the ethereal form. The demon must exert more of its strength to keep the body together and remain in the mortal world, eventually destroying it and being sent back to its plane. There are, however, special individuals who can act as perfect "containers", able to withstand the pressure of having an evil spirit inside them. These are called demonhosts, and are the perfect blend of human and demon in which the spirit can use its powers to full extent without damaging its hijacked form.
Demonic possession acts similar to a virus, which functions by incorporating its own genetic material into the genome of the infected cells. This infection spreads to other cells, destroying the original cell in the process and eventually spreads throughout the body and brain. This form of self replication occurs very quickly and degrades neural tissue in order to complete its life cycle. This leads to the eventual breakdown of the human form, as it begins to show signs of heavy mutation (for example: horns, scaly skin, spikes, etc).
With a demonhost, the viral infection functions the same way, but cannot be discovered by tests. It leaves cells undamaged and cannot be detected as it moves from cell to cell. Being able to leave no trace of itself prevents it from being noticed or discovered by onlookers and scientists as it subtly takes over its host and consumes its soul, which is completely aware of it but unable to do anything about it.
How could a virus operate in this way?