My dermatologist cautions that some doctors treat with steroids for too short a period. Thus if your reaction is following a 24-day cycle and you take steroids for only seven days, the symptoms could reappear before the peak of irritation has been reached. Most botanical field guides distinguish between poison ivy and poison oak. Subtle differences are noted, ones that do not really matter for recognition of the plants. Both have three leaves and produce the oils that make you itch.
Some biologists claim that poison oak is more virulent than poison ivy.
I'm not sure about that part, but I do suspect that under certain circumstances any of us can succumb to these three-leaved plants.