The Flu Bug Machine is a vehicle used by the Common Cold.
Its purpose was to infect the world with the flu virus, so that the Common Cold would not be the only one who was constantly sick. Its first appearance was in Operation: T.O.M.M.Y.. Numbuh 2's little brother, Tommy Gilligan, helped the Common Cold build it in order to seek revenge against his older brother for never letting Tommy hang out with him. However, before the end of the episode, the Common Cold ended up using Tommy as a part of the machine, betraying him.
The machine itself is scorpion-like in design, the gun being located in what would be the stinger. It shoots mucous out of the stinger. The mucous is produced from five children put in tubes on the back of the body of the vehicle. The head of the vehicle houses the cockpit, where the Common Cold controls the machine from.
Miniature Flu Bugs appear as foot soldiers in Operation: V.I.D.E.O.G.A.M.E.. They can also notably walk on walls.