Kyogen

Kyogen was the name of a malevolent non-corporeal life form that infiltrated the computer systems of two Federation starships: the USS Denver and the USS Titan.

When the Denver disappeared along the Federation/Romulan border on Stardate 20609.15, Star Fleet Command sent the Titan to investigate.
The Titan traced the Denver's last known location to a nebula, designation FSG-477. Upon entering the nebula, the Federation light cruiser's systems began to fail.

Kyogen ultimately revealed himself to the crew of the Titan. He claimed to be an energy-based lifeform that inhabited the nebula. He called the crew infections, and then proceeded to use the Titan's own computer to begin exterminating them.

• 34 people died before the crew could eject all three computer cores and then destroy them, killing Kyogen.
  

The Denver suddenly appeared, and the frigate's captain confirmed that they too had been infiltrated by Kyogen.
They managed to expel it by subjecting their computer cores with large amounts of radiation. The Denver then towed the Titan back to Starbase Bravo were they were refitted with new computer cores.

• See : "Ghost in the Machine"

The word "Kyogen" on Earth is Japanese for "mad words" or "wild speech" and is a traditional Japanese form of theater.
It is unknown if this is merely coincidence or how the universal translator interpreted his name. Since Kyogen had infilitrated the starships' computer systems, he may have also chosen that name himself while searching the language database.