Inspiration
My inspiration was a work found at LycanthropeKeith's
Which just went down as I typed this, I saved a copy so I just mirrored his work on my server.
Though it was my inspiration I did not like the total implementation. I thought they were overpowered and mingled virus's with spirits. The whole idea of having a wolf spirit actually be forced to possess a body seems over the line and introduces a host of issues that must be taken into account. Possession of a metahuman by a spirit is something else entirely and unwilling possession, by either party, to my knowledge is not covered in the rules. (if i'm wrong pm me)
Taking today's knowledge of retrovirus's and the information so far given int he Shadowrun sources i had the following thoughts:
RNA virus's take DNA from the host when they enter the body.
Retrovirus's are being used in leading edge research to cure many of the diseases that plague our society by re-writing the genetic code within our bodies.
Couple these two facts with Shadowrun and it is easy to see how the lycanthropic virus came into existence and how it spreads.
Consistency with the Shadowrun Universe
The release of Augmentation, Runners Companion, Disease Rules and additional HMHVV's simplified the thought process for me. To my knowledge i only had to modify one rule. Found on page 86 of the Runners Companion "Shapeshifters cannot be infected with HMHVV or become drakes" changes to "Shapeshifters cannot be infected with HMHVV-I, HMHVV-II, HMHVV-III or become drakes". In all other cases I endeavored to reference the core rules not only for continuity but for ease of play.
The Document
Things i know that may need tweaking
Transformation Times
Transformation TN's
Hybrid form attribute augmentations
Animal form attributes
You can download it here
SR4 Shadowrun Lycanthropy