Poison ivy and poison oak rashes are caused by allergy to the juices of these plants - called Rhus plants. You don't have to come in direct contact with the leaves, roots, or branches of Rhus plants to get the rash. The plant juice can reach your skin indirectly when you touch clothing or a pet that carries the plant juice.

Like other allergies, Thus allergy is acquired; you're not born with it. While some lucky people never become allergic to Rhus plants, most persons become sensitized at some time and remain allergic. Unfortunately, there's no way to desensitize persons allergic to Rhus plants. The many drops, tablets, and injections that supposedly produce immunity are of little value. Some have caused unpleasant side effects. We don't recommend them.

Contagion
Your poison ivy rash is not contagious. The fluid in the blisters does not spread the rash. Rhus doesn't appear immediately after exposure to the plant juice, but only after a time called the latent period. This latent period between exposure to the plant and appearance of the rash may be as short as four hours or as long as 10 days, depending on individual sensitivity and the amount of plant contact. Sometimes more rash appears after treatment has begun. These new patches are areas that had a longer latent period.

Prevention
The only way to prevent Rhus rash is to avoid contact with the plant juice. It is traditional advice to wash with strong soap after poison ivy or poison oak exposure. This does no harm, but in order to prevent a rash, you have to wash within 15 minutes of exposure. If you can do so, simply wash with water and mild soap.