Usually it only spreads where the oil touches. That is why you need to be careful of the clothing you were wearing, etc. You can't get poison ivy from touching someone's rash, blisters or even the oozing gunk unless they haven't cleaned off the original oil. Can you tell I've been through this before? I'm a Scout leader.
By the way, when camping be careful about what you put in the campfire. If you burn Poison Ivy the oil can be carried in the smoke and can even cause a rash inside your mouth and throat. not a pleasant thought.
My son is very allergic. The first time he got it where he couldn't even reach, like the center of his back, when the only place it touched him was his ankle. He could have scratched and caused it to spread that first day, but that doesn't explain the way the rash continued to spread. He'd have had to have taken off his clothes and rubbed his hands all over his body. It spread for days after the first contact. His doctor said that some people have a reaction that spreads through the bloodstream, but it's not common.