The 'poison' in poison ivy, poison oak, and sumac is an oil called urushiol. Urushiol acts as an allergen. Like other allergens, such as pollen or pet hair, not everyone reacts to it, the reaction depends on the dose, and you can become susceptible at any time, even if you never had a reaction before. The most common symptom of exposure to poison ivy is contact dermatitis, which usually manifests as a red, itchy rash with raised areas (hives) or fluid-filled blisters.
You don't immediately break out into a rash as soon as you touch poison ivy. It takes time to react to the allergen. If you have been exposed before, even if you didn't react, you can start to itch with a few hours to a day. If it was your first exposure to any of the urushiol-containing plants, you might not show symptoms for a week or two. Anywhere from 5 hours to 15 days is common. Since you usually won't know you have poison ivy right away, there is an excellent chance you will spread it around. Your clothes can transfer the oil (watch where you throw your laundry or where you sit). Tools and pets are good at spreading the oil, too. You can't spread it from your skin by the time you see a rash... the oil has already been absorbed. Since the reaction depends on how much of the oil was present, indirect exposure to poison ivy tends not to be as severe as what you would get from the plant. You can usually manage the itching with an antihistamine (like Benadryl) and cope with any swelling with corticosteroids. The reaction runs its course in 10 days to 3 weeks, though a really severe rash may take up to 6 weeks to heal.