Either the poison dart idea, OR this:
The same as piggy said, except instead of it being used on a person, it's used on a gun. But it infects EVERYONE who picks up the gun, and the only way to cure it is to use the health station (it wouldn't be an AIDS gun, but rather a Plague gun or something).
EDIT:
I still like my idea for a traitor/detective item called the 'backpack' that lets you carry any additional weapon (primary/secondary/grenade/traitor)
It would be an omni-slot that could be used to not only kill people with a mac10 and then claim that you have a HUGE, but also allow you to be more versitile by carrying a silenced pistol and perhaps still plant a C4 or carry a knife.