FDA Information regarding poison ivy and other poisonous plants:

On this page:
http://www.fda.gov/consumer/updates/poisonivy090208.html

Recognizing Poison Ivy, Poison Oak, and Poison Sumac
Not Contagious
Tips for Prevention
Tips for Treatment
See a Doctor
First comes the itching, then a red rash, and then blisters. These symptoms of poison ivy, poison oak, and poison sumac can start from a few hours to several days after exposure to the plant oil found in the sap of these poisonous plants.

Recognizing Poison Ivy, Poison Oak, and Poison Sumac