A cautionary tale:
Life long immunity ... often isn't life long. I have heard it opined that some folks have relative immunity but can lose it permenatly or temporarily. Still not sure which I am...
I used to walk and darn near roll in the stuff, no adverse effects. If you got tossed off a horse in the middle of a patch you shrugged and walked out.
Surface exposure never bothered me, then one day I got some sap in a about a half inch surface, cut on my wrist (I was clearing the stuff, wearing a boots jeans, a tank top and leather gloves). The 1 inch branch caught and bounced back on my wrist... I know my arms got a brush here and there, but it had never before bothered me. Why worry?
After three days I had a little red rash on my wrist immediately around the injury. I said "Oh look I got Poison Oak"!. After 5 days it began to spread. After 10 days I was in sheer agony. After two weeks it was still spreading.
Prednisone only works sometimes and for some people. My best friend is one of those people and is highly reactive to PO. She camps and hunts and often has world class PO: gets it off the dogs. Prednisone makes it bearable.
Prednisone only works well if you are not reactive to the prednisone. I looked like a heroin addict in need of a fix. The 'cure' was wose than the ailment it was intended to treat.
I am going to observe that the tale about surface oil exposure did not seem to fit my situation: it appeared that the oozing from the original injury was the pattern for additional injury. I was getting spread after more than a week of 100% clean living away from any affected clothes or additional in-person exposure. The areas of spread were where the loosley wrpped bandaged area was toching my body as I slept. This was also despite following
all recommended precautions and cleansing patterns.
Doctors told me I was reinfecting.
I had NO additional exposure, even left the area during that time. I eventually burned all clothes which might even have gotten washed with other clothes which got washed... as the itchies kept spreading and spreading.
Heartfelt advice:
Don't EVER let the stuff break your skin. 
.
I have had no problems since. I have had known surface exposure, but have always treated cautiously
(I also used Dawn~ factoid: at one point the US Navy did experiments to determine the most effective surfactant for various types of oils: Dawn won hands down.)
Never gotten exposure from the cloths since, again with known exposure at times when the oils are at prime levels for 'getting it'.
BE careful out there.
I also did not see mentioned the danger of smoke: the oils can volitalize and KILL you if it gets in yoru lungs.... never
ever get near smoke from Poison Oak.