A doctor explained it to me this way: "We are all born with some degree of immunity. After that, you start to lose your immunity. Don't push your luck, buddy."
You see, I was standing in the stuff up to my knees digging a p.i. plant out with my pocket knife so I could show the thing, roots and all, telling the park visitors that I was never bothered by the stuff. After that, I bought a plant I.D. book and bookmarked the page to show park visitors.