Claudius Imperius Virus
Claudius Imperius Virus was a late Roman Emperor most famous for his failed invasion of Gaul. Virus gathered the VI, VIII, and XIII legions, and from his base at Narbo, moved his troops into Gaul. After facing heavy resistance from the natives, he got as far as Massilia (present day Marseille), before realizing that Gaul was already part of the Empire.
Since then, the term virus has been hijacked by the medical community to describe any inappropriately timed malaise or illness.
Viruii come in two kinds - nice and nasty. Nice viruii are the kind that might buy you a new puppy for Christmas, whereas nasty viruii would make sure the puppy was pre-sheared and stuffed with concise cooking instructions.
A third kind of virus has been hypothesized, the neutral virus, which has no effect whatsoever. An example of this would be getting gangrene in your legs after they'd just been cut off by heavy machinery, or catching AIDS from a necrophile.