Most prevalent where woods meet grassy areas, these plants can be found anywhere—even in a city—so keep your eyes open for clusters of three-pointed leaves (ivy and oak) or seven to 13 leaflets that grow as rows of paired leaves with one at the end (sumac).
Spot it: Any part of the body that comes in contact with the plant's oil may get red and itchy and develop swelling, sometimes followed by blisters. You may not break out all at once; symptoms could actually worsen as many as five days later.
Treat it: While the rash should go away on its own within one to two weeks, you can ease the itching with calamine lotion and an antihistamine. See your doctor if your rash is severe or your eyes, face, or genitals are affected.
Prevent it: The oil from these plants is most potent for 30 to 60 minutes after contact, so carefully remove and wash your clothing, and rinse exposed skin with soap and water.