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This information applies to poison ivy, poison oak, and poison sumac because all three contain urushiol oil. This colorless or pale yellow oil soaks slowly into the skin when one comes in contact with one of these plants.
Contrary to popular belief, the rash does not spread as a result of scratching. Normally the "spreading" observed is due to the variation in thickness of the skin on different areas of the body. The thicker the skin, the longer it takes for the oil to soak in, which may make it seem as though the rash is spreading. It is much more common to have an infection spread across the skin from bacteria underneath the fingernails which is deposited under the skin while scratching feverishly at the general area of the rash. Contact with the nasty ooze or puss from the infected areas is in no way contagious.
The rash is an allergic reaction to urushiol oil and some people (about 15% of the population) are fortunate enough not to have the allergy. For the large majority of people, however, the itching horror can last up to two weeks. There is really nothing you can do to get rid of the problem. You can only try to relieve the symptoms. Here are a few ways to do so:

Prescription
Over-the-counter hydrocortizone topical cream is not strong enough to have any effect on the rash. Prescription cortizone can actually stop the reaction if it is used early enough, but if the rash has developed cortisone will not help. Furthermore, if you do not take the entire prescription (at least six days, some say a full two weeks) the rash can come back even worse.

Over-the-counter
Calamine lotion, or Burrow's solution are commonly used to combat the itch. They are available at most any pharmacy. They also are minimally effective at best. Some people recommend antihistamines as they should help reduce the histamines that are causing the itching just like with most alergies. Most reports are that they are ineffective.

natural remedies
There is a long list of down-home remedies. The most common is a bath with either a cup of Aveeno oatmeal or a pound of baking soda and two cups of linnet starch. There are great claims of 24-48 hour relief with both jewelweed and plantain (a.k.a. buckhorn) but they require fresh sap from the plants which can be hard to find. Plain old water is said to kill the itch temporarily via a cold water compress or a hot shower. For the hot shower get the temp up to just beyond comfortable and stay in it for about ten minutes. It will itch like mad at first, but the hot water supposedly loosens the histamines and gives itch relief for between 8 and 24 hours. Other suggestions are aluminum acetate baths and urine (for external use only!). Also avoid sweating or any form of raised body heat as it will increase the itching. And finally, summon up every ounce of willpower to keep from scratching!