Filovirus
Filovirus is a small infectious agent which can cause the host of the virus to gain a lot of weight in a small amount of time.

The virus is generally transmitted through World of Warcraft players at conventions such as Blizzcon and MLG. There are a few World of Warcraft players with the virus, most of whom are part of the skill-capped team.
Dude, have you got Filovirus? You've gained like 40lbs in the last 2 weeks...