It only takes a matter of seconds for a normal day in Philadelphia to devolve into chaos. At the start of the 2013 action-thriller World War Z, main protagonist Gerry Lane (Brad Pitt) is stuck in traffic when the first zombies come running through the streets, grabbing and biting any human they can get their hands on.
From the start, it’s clear that these zombies are ones you’d particularly not want to mess with: they’re incredibly fast, agile, and can infect humans in mere seconds with a single bite. The virus they carry quickly spreads across the globe, triggering a full-blown zombie apocalypse.
WARNING: Spoilers ahead for World War Z.
Lane, a former United Nations investigator, is one of the few lucky humans to survive on board a U.S. Navy ship. But in order to earn his keep, Lane is instructed to help a group of investigators uncover the origins of the zombie virus and a way to protect survivors against it.
While the origin of the virus is never fully revealed, Lane does discover a cure. As he hops between countries, he notices a few scenarios where the zombies seem to pass people by. Those individuals are either ill or severely injured, leading Lane to the conclusion that the zombies must only select healthy humans as their viral hosts.
A fateful arrival at a research facility in Wales leads Lane to test out his hypothesis. Being the fearless protagonist, Lane injects himself with a mystery virus and stands face-to-face with a zombie.
It sniffs the air for a few seconds, then appears to ignore Lane, showing no interest in biting him. The presence of an infection appears to make Lane smell too sick for the zombie to infect.
There are obviously no disease-sniffing zombies in real life, but does illness have a smell? In some cases, yes. Humans and animals can sniff out how their peers are feeling, though that ability is more often used to heal disease rather than spread it.
There’s a whole science to the smell of sickness, and it’s way more interesting than nosy zombies.
Reel Science is an Inverse series that reveals the real (and fake) science behind your favorite movies and TV.
The Nose Knows
Humans have always relied on their senses to tell when someone is ill. We use sound to listen to someone cough or sniffle, sight to observe pale skin or tired eyes, and touch to feel if someone has a fever. Smell, too, has been linked to sickness since ancient times.
Today, there are a number of conditions that are medically recognized to give off a certain scent: for example, your breath might smell fruity and acidic if you’re experiencing diabetic ketosis, or like rotten eggs and garlic if you have severe liver disease.
And in some extreme cases, people who are super sensitive to smells go above and beyond the scents that most people recognize. In 2019, researchers reported a case study of a woman who was able to detect Parkinson’s disease via smell with stunning accuracy.
The woman, Joy Milne, first noticed her own husband’s scent change as he developed Parkinson’s. She told NPR in 2020 that it wasn’t until going to a Parkinson’s support group that she noticed the people around them all had the same odor — one that smelled greasy and musty.
Initially, Milne was met with skepticism from doctors; it didn’t seem logically possible that Parkinson’s would have a scent at all. But Milne later became the subject of the case study when one of the researchers saw a report about dogs detecting cancer.
While dogs have undeniably more powerful noses than humans, science does show that humans are also sensitive to changes in bodily odors — even if we don’t all have a heightened sense of smell.
Existence is smelly
We’re constantly emitting odors just by being alive. Volatile organic compounds (VOCs) that comprise those smells continuously escape from the skin and breath, filling up the spaces we spend time in.
It might be easy to recognize when someone hasn’t showered, but there’s also some evidence to show that we might be sensitive to body odor for other reasons. In 2014, one study looked into how people reacted to the smell of their peers whose immune responses were heightened (which is something the body does naturally while fighting off an illness).
The research team injected one group of participants with lipopolysaccharide, a toxin known to stimulate immune response, and a control group with saline, which has no effect on the immune system.
The researchers had those participants wear tight shirts to collect their sweat. A separate group of people was later asked to sniff the shirts and report how they smelled. People who had a heightened immune response gave off more intense, unpleasant odors than the control group, and were also described as smelling unhealthy.
Interestingly, neither the experimental or control group created more sweat, which led the researchers to conclude that there must have been a change in the compounds in their body odor. However, they didn’t pinpoint exactly what compounds changed.
In the case of Milne with her ability to smell Parkinson’s, the researchers did find a few compounds associated with the disease, which could lead to the development of new tests designed to diagnose patients.
Figuring out which compounds are associated with certain illnesses or conditions is an ongoing challenge for scientists, and doesn’t always have a simple answer. In studies on dogs, many medical conditions can be consistently and reliably diagnosed via smell. But we don’t always know what the dogs are smelling.
“Most diseases seem to possess complex odors capable of being trained on by dogs but not easily decipherable in terms of molecules,” Andreas Mershin, a biophysicist at Massachusetts Institute of Technology, tells Inverse.
Sniffing out patterns
Dogs have incredible sniffing powers, especially when it comes to detecting cancer. Studies show that our furry friends are capable of sniffing out lung, prostate, skin, bladder cancer and more.
So how do they do it? Mershin, who studies olfaction and co-authored a 2021 report on dogs detecting prostate cancer, says that they’re likely sniffing out patterns rather than individual compounds.
“There might not be specific volatiles, but instead a whole imprint on many metabolic processes affected by prostate cancer,” he says. In other words, dogs aren’t necessarily smelling the cancer itself, but rather sensing the whole picture of how cancer changes the body.
Mershin says that many of the diseases that dogs can detect do not have a known molecular signature, yet they are still capable of consistently knowing when someone is infected. And in the case of early detection, dogs often beat other methods of testing to the chase.
“Dogs have been shown to be able to pick up bladder cancer earlier than any hospital test,” Mershin says. “Covid, too.”
Their impressive noses probably make dogs the closest real-life analogue to the World War Z zombies. In the film, the zombies seem adept at sniffing out people’s health status with an almost superhuman ability, and even detect the presence of Lane’s new infection in a matter of seconds.
At least in the real world, medical dogs use their sniffing powers as a force for good.
World War Z is streaming on Netflix now.
