Poison ivy, poison oak, and poison sumac are the common names for Toxicodendron dermatitis (formerly Rhus dermatitis). This is a contact dermatitis resulting from a type IV hypersensitivity reaction in sensitized individuals to the oleoresin urushiol. Urushiol is found in most parts of the plants from this genus, which is a member of the Anacardiaceae family. The Toxicodendron genus is pervasive throughout the continental United States, southern Canada, and Mexico and is mostly found below 5000 feet of altitude. It can also be found in Asia, Africa, Australia, and New Zealand. It is estimated that up to 75% of the North American population is sensitized, and the condition has no predilection based on age, sex, race / ethnicity, or skin type. Occupational and recreational exposures are prevalent.

These species show regional variations in the nature of the plant including growth pattern, leaf characteristics, and flowering.

After exposure, the rash begins to appear within 1-2 days in previously sensitized individuals; in the newly sensitized, it may be delayed 2-3 weeks. As the resin is very stable, occult contact may occur from contaminated clothing, gear, or vegetation, even after months have elapsed.