Vagonbrei parasite
From The Stargate Omnipedia
Microscopic organism indigenous to the planet Vagonbrei. The parasite gorges itself on melatonin in the human body until becoming engorged, effecting the surrounding brain tissue and provoking a lethal aneurysm.
The only known cure lies with a local lizard, which possesses a natural immunity.
KEY EPISODE
Morpheus - Stargate team members visit Vagonbrei and soon discover an indigenous parasite which was responsible for killing the entire population in its sleep.