This question was posted originally on Quora.com and was answered by Robert Frantz, B.S., M.D. Zoology & Medicine and Healthcare, The University of Oklahoma (1996) Several years ago I developed this troubling rash on the inside of my left arm. It started out small and gradually got worse. It itched slightly and was on a sort of reddish base of skin. My wife asked me about it but I didn’t think too much of it because it was the size of a quarter. A few days later this thing is now the size of a dollar bill and sort of blistered. I wasn’t having any other symptoms so I didn’t believe it to be serious but I wasn’t sure. I mean, could this be cellulitis? Could it be eczema? What if it was an allergic reaction of some sort? So, I start ruling out all this things in my mind. No fever…check. Not on the extensor surface of my arm…eczema…check. Anyway, after about ten days it seemed better. I went to work and in the normal course of my day I went over to consult with one of my colleagues in the ER about lunchtime. I had taken off my lab coat because it was warm. Within about five seconds of seeing me my friend asks “how long have you had poison ivy?” Face palm moment. Our education and training (like all our attributes) can also hinder us. Source