Is disgust purely psychological?

First, the focus here is not the disgust you feel. Since you have admitted to having OCD, we need to look at it from that angle as a symptom of that disorder. The obsession with the fear of contamination is a common factor in OCD. Should you succeed to overcome this particular obsession, it is likely you would substitute it with another. OCD is a mental illness that needs in-depth psych. therapy to be able to manage it. I could tell you ways to help overcome it, because of my background, but it needs to be an ongoing treatment. Seek out a good therapist if this is causing you such a problem that it is interferring with your daily activities.