Biological horror stories are built around the resolution and denouement. Videodrome doesn't end with the guy learning to live with the signal and his twisted form, it ends with him dying to destroy Videodrome!
The joining of host and pathogen, in a sense, creates a new life form, and the host risks losing the self to that thing that now lives within him. Sanity is a measure of how much an Infected character retains their sanity in the face of the gradual deterioration of their body by the pathogen and the grotesque urges it forces upon their mind. As the power of the pathogen in their body grows, the character Sanity inevitably spirals downward.
A character's Sanity can be no higher than 10 − Resonance, and when Resonance overlaps Sanity, a dot of Sanity is knocked off and the character must make a derangement roll.
10 Selfish thoughts. Using one-dot subversions. Witnessing use of other subversions.
9 Minor selfish act. Witness injury to another.
8 Injury to another (accidental or otherwise). Using two-dot subversions.
7 Petty theft. Witnessing death. Breakup of a significant relationship (such as a marriage).
6 Grand theft (burglary). Using three-dot subversions. Witness overt supernatural powers (non-pathogen). Suffering a life-threatening disease.
5 Intentional, mass property damage (arson). Witness a friend, family member, or loved one die. Suffer the trauma of war or similar high-stress situation (such as a terrible car accident).
4 Impassioned crime (manslaughter). Using four-dot subversions. Suffer sexual or physical abuse.
3 Planned crime (murder).
2 Casual/callous crime (serial murder). Using five-dot subversions.
1 Utter perversion, heinous act (mass murder).
An infected might develop a mutation instead of a derangement following an unsuccessful degeneration roll at the Storyteller’s discretion (e.g. the degeneration was brought on by gaining Resonance or using a Subversion). In this instance, the mutation can be “cured” just as a derangement might. If the character’s Sanity increases back to the level it was when he experience the degeneration that preceded the mutation, that mutation fades. Unlike derangements, though, which may or may not return if the character’s Sanity drops again (see p. 93 of the World of Darkness Rulebook), mutations never truly leave the character. The character still feels the effects of the mutation if his Sanity increases, just not strongly enough to affect him on a mechanical level. If the character loses Sanity again, the old mutation resurfaces, and the character must check once again for a new detriment (either mutation or derangement).