[brookn]

Poison Ivy causes an allergic reaction because of the oil in the plant. The oil is tenacious and will stay on your skin and clothes for a long time unless washed off. You could wear clothes during your exposure, wash them and if not washed well could put the same clothes on days later and expose yourself all over again. There is also a risk if you are not susceptible, and say a family member is, by touching them you can expose them if the oil is on you. Also wherever the oil is on the skin can move it to other areas of the skin. Kind of gross, but that is why you hear of people getting it on their delicate regions. I am allergic to poison Sumac, and I was exposed once and rubbed my eyes without realizing it, they swelled shut.
I have no knowledge about the orchids, but snowballs comments about the hairs makes sense.