The Sleep parasite, also known as Morgan Le Fay's curse was a microscopic parasite that infected the people of Vagonbrei with the parasite managing to wipe out the entire human population on Vagonbrei. All animals and birds were also infected by the parasite, however at least one lizard had an immunity to the parasite. It was shown to be activated from a dormant state when it came into contact with live tissue. It then produces Melatonin which causes a strong urge to sleep.
Once the infected person falls asleep, the parasite gorges itself on the serotonin produced growing to many times its size. The infected person then dies from a stroke. (SG1: "Morpheus")