An animal’s body odor, human bodies included, is influenced by genetic inheritance, gender, health, medication, lifestyle, and diet. Major histocompatibility complex molecules, which play a role in the immune system, affect body odor.
Among other things, dietary toxins are exuded onto the skin. Sweat alone is lightly scented.
Body odor takes work, though not human effort. Corynebacterium live on human skin and are plentiful in the armpits where sweat collects. They metabolize the lipids in sweat, raising a stink in the process. Beyond providing raw ingredients, people have nothing to do with how smelly their sweat is. Corynebacterium are common in Nature – soil, water, plants, and food – and are mostly innocuous, though a ne’er-do-well in the genus causes diphtheria.
Lipases are essential to digestion of dietary lipids into most organisms. Genetic encoding for lipases exists even in certain viruses.
The industrious armpit bacteria break down the lipids into smaller molecular by-products, including butyric acid. Butyric acid is found in butter, parmesan cheese, and vomit. Butyric acid is a product of anaerobic fermentation that also occurs in the colon. Butyric acid has an unpleasant odor and an acrid taste, with a sweetish aftertaste similar to ether.
Mammals with good scent detection, such as dogs, can smell butyric acid at a concentration of 10 parts per billion (ppb). Humans can detect butyric acid in concentrations above 10 parts per million (ppm).