A terrible disease that occurs when a tiny chicken gets into the head of a human and lays eggs. The eggs hatch and more tiny chickens are born. Then the new generation of tiny chickens lay eggs, then they hatch and so on until there a so many tiny chickens that they can't fit inside the head. They need to get out, but they're pressing against the skull. The only way they can escape is via the nose/mouth/eyes.

Or, for the less imaginitive - the common cold

Often abbreviated to 'TCD'

the thing that you don’t have when you’re phones percentage is over 20