Many people will say that it does not spread... but those people might not be as sensitive to the oils from the plant than others.
It all depends on how sensitive you are (how allergic you are to it) and how much of the oil you've gotten on yourself. If you got quite a bit on you, and you're very allergic, the oil can seep out of the inflamed area and spread to more skin. This is why products like Caladryl, because it covers the blisters and help to dry them out, absorbing not only the blister fluid, but the fatty-acid/oil form the plant that causes the inflammation.
This is also why washing the area with soap is good - soap will dissolve the oils and help wash it away without getting on more skin. Then make sure to pat the area dry - rubbing will make the irritation worse and possibly spread any remaining oils.
So, a rather long answer for something that I can easily sum up as being "yes - poison ivy can spread" ;)
In fact, I got it once from bumping into someone who had it on their leg - my ankle hit the area, and the next day - bam! Itchy blisters on my ankle. We were indoors. [ MFS's advice column | Ask MFS A Question ]
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