Jeanette Bicknell considers whether physical and moral revulsion are essentially the same:
According to [author of Don’t Look, Don’t Touch, Don’t Eat: The Science of Revulsion Valerie] Curtis, moral disgust evolved out of physical disgust and originally served the same purpose of infection avoidance. We need to interact with others, but we don’t want to risk contamination with their bodily fluids. So we work out a system of manners and rules so that we can interact without contamination. This is the beginning of morality, says Curtis. We are disgusted by social “parasites” just as we are by sources of (literal) disease and infection. Our disgust at immoral behavior makes us shun perpetrators, who in turn become ashamed and less likely to break moral rules in the future.
Psychologist Paul Rozin has defended a different view.
According to him, disgust is deeply connected with the ideas of contagion and contamination rather than with genuine sources of disease. For example, Rozin and his colleagues found that adults would not eat chocolate that had been formed into the shape of dog feces. The disgust aroused by poop was transferred onto the harmless (and delicious) chocolate. According to Rozin, the things that disgust us are not only those that might make us sick, but also those things that remind us of our animal nature. Humans eat, excrete, and copulate like other animals. Yet we are deeply uncomfortable with the idea that we are, in fact, animals, and like all other animals, we’re destined to die. Disgust functions as a “defense mechanism” to keep awareness of our animal nature and our mortality at bay. Once disgust is in place as a “guardian” of the physical body, it can be elicited not only by things that could make us ill, but by very different kinds of stimuli. Each human culture “co-opts” disgust, says Rozin, and projects the emotion onto people and behavior it considers immoral, even if they present no significant risk of disease.
Related Dish on the subject here.