Poison ivy (Toxicodendron radicans or Rhus toxicodendron) was a plant species native to subtropical and temperate environments on Earth whose oil, called urushiol, could cause mild to severe contact dermatitis. Symptoms included rash, inflammation, burning sensations, and/or severe itching. The plant was identified by three lustrous, smooth-edged leaves per cluster protruding from a vine or shrub.
Noah Lessing, a transferred crewmember from the USS Equinox, claimed to have stumbled into a patch of poison ivy in McKinley Park when he was a child. He compared the symptoms to that of the appearance of a Regellian blood worm. (VOY: "Equinox, Part II")