No, not that kind of dirty, ya PERV. Paul Rozin, a psychologist and emeritus professor here at Penn was featured in an article in today's New York Times for his expertise regarding the evolution of disgust as an emotional response.
According to the article, disgust, which up until recently was not fully understood from a psychological perspective, is one of the most universally elicited emotions–more so than anger and even fear. Initially, disgust evolved so we would avoid putting dirty things in our mouths (as if that's ever stopped anyone). Rozin and his colleagues conducted research to elaborate this hypothesis, finding that another reason disgust exists is to separate ourselves from animals--which explains why we (well, some of us) find behaviors like pooping, dying and sex super icky.
The rest of the article reflects on the immunological response to disgust and how it's used to promote cleanliness in advertising. Yay, science! Yay, Penn! And most of all: YAY, POOP.