Cancer Demons can possess other entities on physical contact, but only if a certain area of their occupied object is touched. The demon then uses its host's natural regenerative healing factor to stimulate its own growth, growing an exoskeleton that corresponds with the Demon's type and species. As the demon matures, the possessed organism will grow more powerful, and the infectious exoskeleton will encroach upon the host's body.
However, the demonic process is symbiotic in nature. The demon will not posses the mind of its host, and the only real effect on the host's health is that the carapace eventually melds with its host's skin, and therefore becomes exceedingly difficult to remove.