Being allergic to poison ivy? No, I wouldn't think so, since it's so common that that would DQ like 2/3 of the population from military service.
Don't worry about DoDMERB until you get to that point. I was like you, all worried, but when it came time to do my physical it was SO simple and fast I couldn't believe it. Also, post questions related to health in the DODMERB subforum