- It seeks out the dominant character at the time and cannot be resisted.
The "dreaded Mary Sue disease," Vambiolaria, was created in the Harry Potter fandom by a fan-made spell gone awry. It manifests as an infectious purple mist and pinkish light that "seeks out the dominant character at the time" and causes them to become Sued.
When Vambiolaria strikes, it first affects the environment around it in a manner similar to Dementors: in addition to the purple mist and pink light, the temperature in the room drops, any fires go out, and the victim will feel shadows closing around them. The victim will experience nausea, vertigo, and splotches of purple and yellow in their vision. Finally, they pass out.
When they wake up, they are Sued. They will have a tendency to be over-described, speak in horrible grammar, cause members of the opposite sex to swoon, and wear swishy dresses (or the male equivalent) despite not owning any such thing. They may also acquire magical Sue artifacts out of nowhere, again despite not owning them or even being anywhere near them in the first place.
Vambiolaria is easy to identify by its initial presentation, but if no one sees this happen, it may be difficult to distinguish from other types of Sueification. Fortunately, the victim may have some awareness that something is wrong (as evidenced by angsting about how they're cursed, or are a curse, and attempting to withdraw from people), and questioning them may prove critical to proper diagnosis. The interviewer will of course have to sift through the bad grammar and angst to get to the truth.
To prevent Vambiolaria from spreading, it may be necessary to sedate the victim. At HFA, Sirius Black whacked Sued!Meir Brin with a solid bronze celery cricket bat, but other methods may prove equally effective.
The cure is a preparation of Kuswort which includes some kind of powder, a dark liquid, frog bowels, and pickled crow bladders. When ready, the potion is lobbed by the handful at the victim, instantly curing them.
The victim may experience some shock at what has happened to them and the fact that there are crow bladders all over their face. They will then enter a recovery period, during which time they will suffer cold-like symptoms and episodes of embarrassing clumsiness, such as sneezing messily all over someone.
As a Weapon
As of 2011 HST, the League of Mary Sue Factories has managed to create the Vambiolaria Bomb, which is very much like a nuclear warhead contained with the virus. Prior to that the Sues had resorted to Glitter Bombs, which cause much less damage but can still inflict various Mary Sue maladies on those in range.