It's in the food we eat: in the cheese that shares chemical properties with bad breath and stinky feet and in the bread that is leavened by microbial excretions. It's in nature: in the viruses that make us sick, in the monstrous shape of reptiles and deep-sea fish and in the terrible parasites that torment them.
Of course, we ourselves might be the greatest source of grossness. We carry it with us, in our blood and guts. Anytime someone's insides end up outside, you are definitely in the presence of the gross.
It seems obvious that our repulsion from the gross is rooted in our fear of death. We may pretend that we are our own masters, but deep down we suspect we are really the victims of nature and fate. The things we spurn as gross are the things that rub that dismal knowledge in our face.