Poison ivy, poison oak, and poison sumac, which are genetically related to each other (they all belong to the genus Toxicodendron ("poisonous tree" in Latin), also known as Rhus), cause more cases of contact dermatitis than any other family of plants. These plants can be found worldwide, but are particularly common in the United States. Poison ivy is usually a climbing vine east of the Rockies, but is a small shrub west of the Rockies. Poison sumac is usually found in boggy areas in the American South. Poison oak is most common west of the Rockies: it is found in 2/3rds of counties in California, and it thrives anywhere from sea level to 5,000 feet altitude. (Of course, there is a Poison Oak Festival in Calfornia each year, with prizes for best arrangement, best poison oak accessory, and biggest branch or single leaf.) Toxicodendron plants are relatively rare in deserts, especially southeastern California, Nevada, southern parts of Arizona and New Mexico, and western Texas.
Poison ivy, oak, and sumac all produce an oily substance called urushiol. Urushiol is actually several chemically similar compounds; the proportions of these compounds are different in different Toxicodendron plants. Urushiol is also found on the skin of mangoes (but not inside the fruit), in the shells of cashew nuts (but not in the nuts themselves -- and cashew nuts are almost never sold unshelled), and sap from the Japanese lacquer tree from which is made the lacquer used to finish Japanese and Chinese wooden furniture and decorative pieces. (The dry lacquer is usually not allergenic.) Urushiol is found in the roots, stems, bark, fruit, and leaves of Toxicodendron plants; it is released only when the plant is damaged, but raindrops can damage the plant enough to release urushiol, as can friction -- including friction from someone brushing against the plant -- and drying of the plant at the end of the season in autumn. Since the woody parts of the plants contain urushiol, it's possible for you to be exposed to it and develop the rash any time during the year. Urushiol is oily, and you can spread it from the original location to other parts of your body with your fingers or with contaminated clothing.
It can take anywhere from 4 hours to 4 days for symptoms of Toxicodendron dermatitis to appear. The first symptoms are redness of the exposed skin with severe itching; the red skin progresses to papules (bumps), vesicles and bullae (bumps full of fluid). Considerable swelling may occur if the urushiol comes in contact with skin on your face or your genitals. Sometimes the urushiol oxidizes and turns black on your skin; these black spots are very difficult to wash off and can cause more severe irritation than "wet" urushiol. The rash and irritation lasts from 1 to 3 weeks if not treated. Occasionally bacteria (most commonly Staphylococcus aureus or Streptococcus pyogenes) will infect the damaged skin. Affected skin may darken, especially in dark-skinned people; this hyperpigmentation may last for months but eventually fades without being treated.
The best treatment for Toxicodendron dermatitis is not to be exposed to the urushiol in the first place. Since all parts of a Toxicodendron plant contain urushiol, the plants can't be handled safely even in the dead of winter; urushiol can also penetrate clothes, rubber gloves, and latex gloves. (Heavy-duty vinyl gloves are impervious to urushiol. Obviously, you must be careful removing the gloves after use to avoid getting urushiol from the outside of the gloves on your skin. Urushiol can also be spread by particles of smoke from the plants, so burning poison ivy may spread the urushiol throughout your neighbourhood.)
If you do come in contact with a plant, you should take off any clothing that touched the plant, then wash your skin with mild soap and water. The quicker you wash yourself, the better: after about 1 hour washing doesn't remove the urushiol. Be careful to clean on and under your fingernails, since urushiol left there can easily be transferred to other places on your skin.
If you develop a rash even after washing, oatmeal baths may help, as may calamine lotion. Oral antihistamines may reduce itching, but antihistamine lotions and cream may actually make things worse by making your skin more sensitive. Steroid creams may help if you apply them before the vesicles appear, but they have to be strong (over-the-counter hydrocortisone isn't strong enough) and you should not use strong topical steroids on your face or genitals unless your doctor tells you to do so. Oral steroids may be needed with severe Toxicodendron dermatitis, or if your face or genitals are affected, but should not be used unless your doctor feels that it's truly needed. (Steroids have many bad side effects if they are overused or improperly used.) Antibiotics are useless for ordinary Toxicodendron dermatitis but may be necessary if bacteria infect the Toxicodendron rash.