Dan,

I believe your on to something.  Unfortunately, science seems to always look at the unhealthy subject and never the healthy subject.  Why is the healthy subject not having such a bad reaction versus others that all but die from it.  I think there is something to do with blood types as well.  I also believe that if they checked for viral factors they would find that people more susceptible to fighting viruses also have a more severe reaction to poison ivy and other allergy causing agents.  Not to mention sometimes people actually have a combination of a virus and have also come into contact with poison ivy.  Now there's a real challenge to figure out...