Lycanthium
Lycanthium is a type of chemical in an infected persons saliva. If such a person were to turn into a lycan and bite a human, that so called person would either turn or die due to the chemical. The Lycanthium in a persons genes would begin to break down and build back up the infected one's cells so that person can transform on comand swiftly and safely; giving them the ability for their bones to shift, grow and/or shrink during the transformation.
"The abnormally large wolf then clamped its jaws around my arm, little did I know the Lycanthium in its saliva would change everything..."