Head Pigeons

Head Pigeons is a "disease" that was mentioned only in the episode "Dark Harvest".

The disease's main symptom and cause is a pigeon or pigeons nesting on one's head. Treatment entails visiting a nurse, or other trained medical professional. Eventually, the bird flies away, and the victim is deemed cured. While the disease appears to be more silly than damaging, Zim's head pigeon stayed on his head throughout the entire episode in which it appeared.

Ms. Bitters also implied the disease was contagious, ordering Zim to leave before it spread to the other Skoolchildren, although this seems unlikely to be the case in the timeframe of Invader Zim.

Facts of Doom

The name of the fan-driven revival campaign Operation Head Pigeons (later Operation Head Pigeons 2.0) is a homage to this gag.
The Head Pigeon was seen in the background during a shot shown in the Invader Zim: Enter the Florpus! teaser trailer.