Once the oil from the poison ivy is washed off (soap and water works fine), the rash will not spread no matter how much you scratch it. However, an infection can set in if the skin is broken. An oil in the poison ivy plant binds to a protein in human skin forming a haptan (protein-chemical immunogen) to which many humans develop an allergy that results in a rash. The severity of the rash can change with age in a single individual. Keep it clean and use a disinfectant cream. As a kid, I got it real bad (even closed up my eyes once). I never could keep out of the woods. Still can't but I need to rollin it now to get a reaction. I do wash up with soap and water now if I think I was exposed.