An old-time saying warns that if a plant has leaves of three, let it be. This bit of folk wisdom is meant to warn people about poison ivy, a noxious vine in the genus Toxicodendron found throughout much of North America. Poison ivy contains a chemical that can cause a vicious allergic reaction when it comes into contact with skin. This rash consists of oozing blisters that itch and hurt.

Some people think they’re immune to poison ivy’s charms. They’re probably wrong. The oily substance it contains, urushiol, tends to cause more intense reactions over time and it affects up to 90 percent of people. Luckily, if within a couple hours of exposure you wash off the oil with soap—and,