I get it second hand... Someone caries firewood with it, then sits in a chair. Later I sit in the chair and poof, I have it for weeks. For me it usually ends with steroids as it doesn't go away. (Last 5 times I've had it has been this way.)
Then on a day where I couldn’t possibly have gone near any ivy I ended up with a couple of spots on my fingers. And then again a week later before I realized it was probably on the boots. Fortunately just a minor inconvenience, but it’s crazy how the oil sticks around.