Why do people get poison sumac real bad?

Rhus dermatitis: Reaction to the toxic oil, can stay on clothing up to 10 years if not washed in hot water.

Subtle reaction: This reaction is triggered after a sensitizing oil, the rhus antigen, transfers to the skin after direct or indirect contact with the sumac plant.(or poison ivy,oak) it does not produce immediate effect but itching usually start by 6-8 hours & breakout by 18 hrs. If not removed by a good soapy washing, the oil can be spread by the itching to any site the fingers then touch.