Poison ivy rash is caused by a sensitivity to an oily resin called urushiol (u-ROO-she-ol), which is found in the leaves, stems and roots of poison ivy, poison oak and poison sumac.

At least 50 percent of the people who come into contact with these plants develop an itchy rash. The most dangerous type of exposure occurs when the plant is burned and the smoke is inhaled, which can affect your lungs.

Mild cases of poison ivy rash require no medical treatment. For more severe or widespread rashes — especially if it’s on your face or genitals — your doctor may suggest taking corticosteroid pills, such as prednisone, for a few weeks.