Brain worm/Legends

"Watch as my child enters your Jedi friend."

- Karina the Great

Brain worms were worm-like parasites from Geonosis that had the capability of controlling minds. They hatched from yellow eggs, and were known to be used by the Geonosian Queen Karina the Great during the Clone Wars on her Geonosian minions. The worms would sometimes enter the host via the nose, and were able to control dead Geonosians.

Brain worms reacted negatively to cold temperatures.

History
"-The nose or the ear, which do you think it will enter? -I don't think she wants to find out."

- Obi-Wan Kenobi and Anakin Skywalker

During the Second Battle of Geonosis, Queen Karina the Great used the worms within her dead minions in the Progate Temple to endlessly pursue the search party sent by the Republic to find Jedi Master Luminara Unduli. The search party was led by Anakin Skywalker and Obi-Wan Kenobi. Queen Karina intended to use them to control the Jedi, but didn't succeed in inserting any into them.

After the battle, a surviving worm hatched and crawled into one of the clone troopers of the Tango Company resting outside the Progate Temple. As the company headed to the medical frigate TB-73 going to the Ord Cestus medical station, the infected clone brought more eggs in his backpack and eventually infected the entire clone crew. The clones took control of the ship, attacking its other members, and proceeded to infect Barriss Offee. Ahsoka Tano ruptured the ship's coolant system, weakening the worms. Subsequently, Ahsoka and the infected Barriss dueled, but the cold finally got to the worm in Barriss, and Ahsoka killed it as it was crawling out. Barriss and all clone troopers aboard the ship besides Trap and the pilots were saved. Trap was killed by Barriss in self-defense, and the pilots were killed by infected clone troopers Ox and Edge.

Anakin Skywalker later speculated that use of the parasite might explain how the Geonosians were able to recapture their planet prior to the Second Battle of Geonosis.