The rash itself is not contagious, and the fluid in the blisters does not spread the rash. Poison ivy appears as soon as four hours or as long as 10 days after the exposure, depending on individual sensitivity and the amount exposure. As the rash appears, any sensitivity a person had begins to increase. One starts to react to the slightest traces of a few molecules on the skin. This causes the rash to appear to be spreading, even after treatment has begun.