Basically, Mats Olsson, a researcher at Sweden's Karolinska Institutet, thought it was weird that sick people sometimes smell gross. That thought was the motivation for a recent study that indicates that humans have developed the ability to detect a toxin in other humans thanks to a keen sense of smell.
When introduced to certain toxins or diseases, the human immune system kicks into gear, causing a spike in the level of cytokines (+1 for fancy science word). Olsson wanted to ascertain whether or not people would be able to detect which of their fellow humans were infected based solely on their smell.
Guess what... sick people have terrible smelling sweat, apparently.
Olsson and his team had eight healthy people visit the laboratory to be injected with either lipopolysaccharide (LPS) — a toxin known to ramp up an immune response — or a saline solution. The volunteers wore tight t-shirts to absorb sweat over the course of 4 hours.
Importantly, participants injected with LPS did produce a noticeable immune response, as evidenced by elevated body temperatures and increased levels of a group of immune system molecules known as cytokines.
A separate group of 40 participants were instructed to smell the sweat samples. Overall, they rated t-shirts from the LPS group as having a more intense and unpleasant smell than the other t-shirts; they also rated the LPS shirt as having an unhealthier smell.