The Bane of Poison Ivy

I was working in my yard on Sunday afternoon and unbeknown to me came in contact with poison ivy of which I am very allergic.  I am now paying for it, although I must say that I have had it far worse than what I have now.  I would call this annoying but it is not driving me nuts (I'm already there so how could it?)

What I find a complete mystery are the places that react.  I know from what I have read that it an only spread through the skin's contact with the sap (whose name I can't remember but know that it begins with an "u.").  But how it can appear in otherwise isolated areas seems bizarre to me. You would think that it would display a trail, a pattern.  For instance 99 percent of the allergic reaction has occurred on my right side, but there is a small outbreak on my left bicep.  It doesn't make sense to me.  In any case, I would have to say that poison ivy rates right up there in I- don't-know-why-God-created-them with mosquitoes!