I know some people who swear they can get it by looking at it, or maybe a neighbor was burning it and they got it from the smoke, or it was just in the air that day. But apparently you do have to touch it. Actually any part of it; leaves, stem or roots. Dead or alive. Or if your dog was romping in it you can get it by petting him. That works too. I always thought it spread on the skin by scratching it but not so. It appears to spread but it's just different timing in the allergic reaction.
Urushiol is the bad ass chemical in the sap and about 85% of people are allergic. Most of us have heard the one about the person who was sure they weren't allergic. Or the one about the poor kid who couldn't walk away from the double triple dare ya.
It grows all over North America except Alaska and the southwest. It doesn't grow in deserts or above 5000'. Along the eastern seaboard if you stop by the side of the road to take a leak you're probably wading into it. So if you like to hike around outside it's a good idea to know what it looks like.
a healthy patch
"leaflets three, let it be"
I've always liked this song: The Coasters Poison Ivy, 1959