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'

When you eat dirt for the first time and leave your family to build a shack in the woods