My father spent a day tearing out poison ivy vines around our house. At the end of the day, he took a shower and scrubbed himself with brown soap.
Instead of getting rid of the poison he spread it all over himself and was sick in bed for a week with a horrible case of poison ivy.
The moral is to be careful with the stuff and be aware that you can spread the oil by washing with soap and warm water. The warm water opens the skin, the soap emulsifies the oil, and the scrubbing pushes to oil into the open skin.
After the oil has been absorbed or washed off, and you HAVE a rash, a hot shower can ease the itching. Many recommend heat for relief from the itch.
(Editor’s note: This happened to my father though I barely remember it. My mother told me about years later.)