First appearing in the 1977 Monster Manual
sourcebook of 1st Edition Advanced Dungeons & Dragons
, the ear seeker
is a tiny "insectoid" parasite
. Adult ear seekers feed on decaying vegetable matter, most notably wooden dungeon doors. However, when presented with the ear of a warm-blooded character attempting the Hear Noise skill, they will crawl in to lay d8+8 eggs, which will hatch 4d6 hours later.
Humanoid physiology features lots of blood flow through the brain - remember the survival tip about wearing a hat because so much body heat is lost through the head? Ear seeker larvae burrow from the ear deeper into the skull, attracted by the heat and nutritious brains. The original description made ear seeker infestation 90% fatal, although the latest rules permit a Fortitude save at DC 18. These later rulings also describe permanent loss of Intelligence and Wisdom points even with a successful Fortitude save, and specify a period of 1-4 days before death (or recovery). If the infestation is anywhere near as painful as a strep infection of the ear, the character will certainly be in no condition for adventuring.
A Cure Disease spell destroys the eggs or larvae if cast before the victim dies. Other healing magic will be required to restore lost Intelligence or Wisdom.
Ear seekers are difficult (though not impossible) to justify in play. They're too lethal to inflict on unsuspecting low-level characters willy-nilly, but once the characters have access to the Cure Disease spell, they're a mere nuisance. Furthermore, in a world where people make a habit of listening at wooden doors known to bear potentially lethal parasites, it's fair to assume that any sort of spy training would include a safety tip like "use a cup to amplify sound vibrations through doors and simultaneously protect your warm nutritious brain".