Body odor
Body odor was a scent naturally given off by humanoids due to their pheromones. 

Locator bombs were attracted by one's body odor. ()

Perceptions

Humans 
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 , noting that Ensign Socorro smelled like a Nausicaan's adrenal gland after she exercised. ()

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. ()

Klingons 
Klingons were noted for having a particular scent. 

According to  in 2151, "The only thing worse than spending three hours in a decompression chamber with a bunch of Klingons is doing it twice in one day. I smelled things in there I hope I never smell again." ()

Following the departure of Chancellor Gorkon and company from the  in 2293, Yeomen  and Samno commented on how "they all look alike," adding furthermore, "What about that smell? You know only the top of the line models can even talk…" ()

According to Barry Waddle in 2373, Klingons were "foul-smelling barbarians." The comment, made in the presence of Worf (a Klingon), was politely reiterated by O'Brien, who said that "I wouldn't take it personally, Worf." Julian Bashir chimed in, "I rather like the way you smell," with O'Brien continuing, "Yeah, sort of earthy, peaty aroma,", with Bashir finally adding, "With a touch of lilac." () 

While taking a bottle of alcohol from an drunk holographic Klingon in 2374, Neelix suggested that he share or he would take it by force, because he could "still send your soul to the next world and leave your body here to rot. Though from the stench of it, I'd say it's begun to already." ()

In 2377, after the  had taken aboard several Klingon passengers, Neelix told Captain Kathryn Janeway that he overheard "some of the Starfleet people have been complaining about the smell," and that "Personally, I find it appealing. A kind of a musky aroma." Janeway, nonetheless, sympathized with her crew and offered to "adjust the environmental controls to filter out the musk." ()

External link

Physiology
Medical conditions