A. If you get a poison ivy reaction this spring or summer, you're not alone: It's estimated that up to 50 million cases occur in this country each year. Poison ivy is a plant that grows as a shrub or vine and produces a resin that can cause an allergic reaction in skin. Not everyone is susceptible: About 50 to 70 percent of people are sensitive to the resin — with about 10 to 15 percent of those people highly sensitive — while “35 percent have no reaction at all,’’ says Oon Tian Tan, a dermatologist at Massachusetts Eye and Ear. “It's very dependent on your immune system.’’

The resin is produced throughout the plant, not just on the leaves, so it's possible to have a reaction from touching any part of it. Tan says that sensitive people will sometimes react when leaves are burned nearby. The rash takes 12 to 72 hours to appear, and includes redness, swelling, severe itching, and blistering of the skin. Once it begins, a rash can last for days, or even weeks. It can shift locations on the skin and subside and then reemerge. Emmy Graber, a dermatologist at Boston Medical Center, says people often believe this means they are spreading the rash, but “it's not true that you can spread it by scratching.’’

It's also not generally transmitted from skin-to-skin contact, Graber says. Instead, it can persist on clothes, objects, and in pet hair, so a rash is often transmitted from a dog, or repeat contact with gardening gloves or an item of clothing. If you suspect you've come into contact with poison ivy, it's important not only to take a shower but thoroughly wash any items that may be contaminated.

Mild cases can be treated at home. Graber recommends using an over-the-counter hydrocortisone cream to soothe itching; bath powders and soaps with oatmeal can also help.

Send health questions to globeanswers@gmail.com.