Poison ivy
From Memory Alpha, the free Star Trek reference.
Poison ivy (Toxicodendron radicans or Rhus toxicodendron) is a plant species native to sub-tropical and temperate environments on Earth whose oil, called urushiol, can cause mild to severe contact dermatitis. Symptoms include rash, inflammation, burning sensation and/or severe itching. The plant is 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 when he was a child. He compared the symptoms to that of the appearance of a Regellian blood worm. (VOY: "Equinox, Part II")
