Dream Riders were a small group within Kendarayan society who were able to bring themselves to a near-death state, and to move back and forth between the barrier of life and death. This was apparently due to a unique nucleotide within these individuals' cellular structure. Dream Riders tend to keep their abilities secret, for fear of being hunted by others in their society.
Sellassars, the Kendarayan envoy sent to meet with representatives of the Federation in 2372, was a Dream Rider. While en route to the summit aboard the USS Enterprise-E, he inadvertently caused a rash of comas when his dead skin cells, in combination with the Barth that covered his skin, came in contact with crew members. (TNG - Slings and Arrows eBook: That Sleep of Death)