Body odor was a scent naturally given off by humanoids.
Physical exertion could lead to the production of perspiration, which had a distinctive smell. In 2151, Doctor Phlox, fascinated by aspects of Human behavior and biology, discussed his observations with Trip Tucker, noting that Ensign Socorro smelled like a Nausicaan's adrenal gland after she exercised. (ENT: "Fight or Flight")
Vulcans found the scent of Humans so unbearable that they sometimes resorted to taking a nasal numbing agent in order to be able to spend long periods of time with them. By 2151, T'Pol had partially acclimated to the smell. (ENT: "The Andorian Incident")
In 2373, while using the alias of Barry Waddle, Arne Darvin referred to Klingons as "foul-smelling barbarians". Julian Bashir and Miles O'Brien told Worf not to be offended, assuring him that he had an "earthy, peaty aroma" with "a touch of lilac". (DS9: "Trials and Tribble-ations")
While getting drunk with several holographic Klingons in 2374, Neelix suggested that he could kill one of them and leave his body behind to rot, adding "from the stench of it, I'd say it's begun to already." (VOY: "The Killing Game")