I enjoy learning about topics that make people uncomfortable and gross them out, too. But my interest is more for the sake of the discomfort specifically-- I want to know why things disgust people, and how that translates into disgust-avoidant beliefs and behaviors. Sometimes it's obvious-- feces smells bad. It's not good to eat, have on you, or be around in general. Any of these things could make you sick, and being nauseated is both a sign of being sick and a response to things that could cause sickness. Rotting food, same story. Decaying corpses, same story. But we have developed elaborate responses to these things way beyond what would be necessary to simply keep ourselves safe from contaminants. The anthropologist James Frazer articulated two forms of "sympathetic magic" which have been found to apply very closely to disgust-- the first being the law of contagion, which is that a thing takes on the properties of another thing by having contact with it. Homeopathic magic, or "like affects like," suggests that the resemblance of thing A to thing B can cause one to act similarly to the other. So in the first case you have the obvious example of, say, a dead fly in a glass of water. It doesn't matter how much water is in that glass; you're not going to want to drink it because of that single dead fly. In the second case, you have fake dog poo, and such things as brownies shaped (and colored) to look like dog poo. Even with the full knowledge that what you're holding in your hand is in fact a tasty confection, you're very likely to look at it with some skepticism.
Of course, you're not guaranteed to look at it that way-- people have different levels of squeamishness, and we can become accustomed to something to the extent that it no longer grosses us out even though it once did. I'm pretty sure that I will never become comfortable with watching a surgery being performed, but some very necessary people-- surgeons-- either began with a comparatively low level of disgust for the whole experience or adapted sufficiently to be able to handle it. The disgust you feel when looking at an open wound is classified as a response to "body envelop violation," which is pretty much just as it sounds. The body has been opened in some way, which presents us with the alarming sight (and occasionally smell) of blood and guts, which is frightening because our sense of empathy compels us to identify with the person who has been injured (in the case of surgery it's obviously not an injury, but this distinction matters not at all to my amygdala) and consider that we also can be hurt, suffer, and even die. A parasite is disgusting for its similar functioning as violating the body-- parasites invade for the purpose of benefiting themselves at the hosts's expense, up to and including death. "Animal reminder" disgust is the emotion we can experience when we think of ourselves as flesh and blood creatures different only in shape from the millions of other forms of furry, scaly, scratchy, or slimy creatures that crawl, fly, sprint, or swim across the planet. You can see this disgust in a person who adamantly avows that his or her ancestors were not monkeys (apes, really) when confronted with Darwin's tree of life theory of evolution. If we're animals, the thinking goes, consciously or unconsciously, we're not special. There is no particular reason for us to be unique in any way. This thought is not at all disquieting for someone knowledgeable about evolution, but can be downright stomach-turning for someone who has been raised to believe that our existence as humans is a result of a special act of God. To say that we are otherwise seems...impure.
In concerns about purity, it's easy to see where disgust as a physical response translates into a moral emotion. Going back to male homosexuality for example, it's where the feeling of discomfort is interpreted as an unease in the face of what is obviously a violation of the laws of nature, the way God made things. If you want a good analysis of how disgust as a moral emotion plays out in the theater of public opinion and even makes its way into legislation, I'd recommend Martha Nussbaum's excellent book Hiding From Humanity: Disgust, Shame, and the Law. In it she recruits study of psychology, psychoanalysis (unfortunately) and the philosophy of John Stuart Mill to examine how moral disgust as a justification for political stances and law-making has led to oppression of minority groups and some very backwards positions on bioethics over the years, as a barrier to compassion, understanding, and acceptance of new science and technology. If you want to read about the most up-to-date psychological understanding of how disgust works as a moral emotion, especially through means of association, I'd suggest checking out the publications of psychologist David Pizarro, whose TED talk on that very topic you can watch here:
You'll notice that he cites Nussbaum, as well as psychologists Jonathan Haidt, Clark McCauley, Paul Bloom, Roy Baumeister, and the mack daddy of disgust research Paul Rozin. They are all interested in how intuitions shape moral reasoning generally, and how disgust does so specifically.
Pizarro cites studies he himself took part in, as well as those of other scientists, to show that people with conservative political leanings are much more prone to disgust, and that it can actually be predictive of their voting behavior-- specifically regarding gay marriage, gay sex in general and other sexual issues. Pizarro actually found that arousing disgust in people caused their political judgments to shift toward conservatism. So not only do conservatives appear to be people who are more susceptible to disgust generally, but you can apparently make a person more conservative by exposing them to sensory input that they find disgusting-- in this case, by making the room they were in smell gross. He even found that prompting people to take precautions involving keeping clean (such as reminding them that washing their hands can help to prevent flu infection) achieves the same effect. Yes, telling people to wash their hands made them think more conservatively in their moral reasoning.
For this reason when Jonathan Haidt articulated his domains of moral emotion, he placed the domain of Purity/Sanctity (along with Respect for Hierarchy and In-Group Loyalty) squarely in the category of conservative thinking. I'm thinking that while this might be entirely valid, there are some things which can be counted upon to arouse disgust in liberals but not conservatives which have not been evaluated in these studies, which have mostly focused on sexual practices, body envelope violations, and excrement. But that is for a post in the future.