According to the beliefs of the Nechani, the nesset were able to travel from this world into the spirit realm where they served as gatekeepers. This appeared to have basis in fact, as being bitten by them was part of the ritual to cross into a shrine, and they produced a toxin causing the natural immunity the Nechani monks to cross the biogenic field unharmed. The breakdown of the toxin also produced a hallucinogenic effect.
In 2373, Captain Janeway put her hand into a basket with a nesset and let herself be bitten, in order to save Kes' life. The nesset bite caused a poisoning that led to a burning sensation in Janeway's chest, but she was in no immediate danger. (VOY: "Sacred Ground")