Brain Worms are a type of alien parasite that Zim made a passing reference to in Walk of Doom. When told he had to pay to ride the city bus, he asked the driver if she had 'the brain worms'. It is unknown if such worms actually exist, or if it was just Zim's typical brand of dramatism, but it is implied that being inflicted with the brain worms results in a serious impairment of intelligence and/or reasoning capabilities. The "brain worms" are never mentioned again after Walk of Doom.