Poison oak is a plant that causes an itchy, blistering rash when you touch it. The plant produces an oil called urushiol on its leaves, stems, and roots. When this oil touches your skin, your body reacts with red bumps, swelling, and an intense itch that can last for weeks.

Poison oak grows as a shrub or vine in many parts of North America, especially along the West Coast. Its leaves grow in groups of three, which is why hikers remember the warning: “leaves of three, let it be.” The leaves change color with the seasons, turning from green in spring and summer to yellow, orange, or red in fall.

The tricky part about poison oak is that the rash doesn't appear immediately. You might brush against the plant while hiking and not realize it until a day or two later when the itching starts. The rash itself isn't contagious, but the oil can spread from clothing, tools, or pet fur to your skin. Even touching poison oak in winter, when it has no leaves, can cause a reaction because the oil can remain on the bare stems.

If you know you've touched poison oak, washing the affected area with soap and cool water within an hour can help prevent or reduce the rash.