A magic virus is any lingering spell that has been created to function solely within the body of another entity, generally a living one. Magical viruses are extremely difficult to create, being of such a high caliber of arcane complexity that no mage alive today knows of how to properly make one that lasts indefinitely.
In that sense, it is a lost art, rumored of in some books but taught in none. Magic viruses were considered a great taboo in the past, and mages who practiced the creation of them were imprisoned or killed and their teachings wiped from history.
Magic viruses are self-perpetuating spells that cause magical changes in their host, and can be passed down through one or several means.
Only their modern-day effects show the potency that they possessed. Some present magical viruses include: