Seek medical attention immediately. Although there are thousands of plants that mildly toxic or even deadly there is such a thing as contact poisoning. Contact poisoning is where you actually come in contact with a plant that will make you ill by touching it. Some of the more common plants responsible for contact poisoning are Poison Ivy and Poison Oak. Others include but are not limited to Poison Sumac, Nettle, and even some blue green algaes that are found in shallow streams.
Many chemicals are available over the counter to help with contact poisoning.
Some of these drugs include calamine and cortisone. Another application of a wild astringent if caught in the wilderness without benefit of chemical drugs is effective also. Dogwood and Chestnut bark can be boiled and then applied to the areas of rash to allievate itching and spreading of the poisoning. The best way to actually treat the areas affected is to make sure the area is clean and washed carefully. Apply which ever available topical ointment or natural astringent to the effected area making sure to use a clean cloth or cotton to every area touched.