The sight of dog dirt or a waft of rotten eggs is enough to turn anyone's stomach.
While no one enjoys the nauseating feeling of disgust, research suggests this gut reaction may have evolved to keep people safe from infections.
Scientists from Washington State University asked 75 people living in rural Ecuador to rate their level of disgust at the thought of touching a dead animal or stepping in droppings.
Results reveal those who experienced the highest degree of revulsion had lower levels of inflammatory markers in their blood that are associated with infections.
This natural instinct may therefore help ward off food poisoning and diseases spread via animal waste.
It will likely do little to protect against the coronavirus, however, which lingers unseen in the air and on surfaces, often after being expelled by an asymptomatic individual who people do not know to avoid.
Charles Darwin, the father of evolution, suggested humans developed feelings of disgust to discourage them from eating spoiled food.
It was unclear, however, whether individuals who experience a higher degree of revulsion develop fewer infections.
To learn more, the Washington State scientists analysed 75 indigenous people – aged five to 59 – from 28 homes in Ecuador.
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These communities often live hand-to-mouth, with a "high pathogen burden", the scientists wrote in the journal PNAS.
The participants were asked to rate their level of disgust when imagining hypothetical scenarios. One of which was drinking a fermented corn drink made by someone with rotten teeth who had chewed the grain before spitting it into the water.
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"As predicted", higher "disgust sensitivity" was "strongly" linked to reduced immune markers in the blood against bacteria or viruses.
"The higher the level of disgust, the lower the level of their inflammatory biomarkers indicative of infections," said study author Dr Aaron Blackwell.
"While the study shows disgust functions to protect against infection, it also showed it varies across different environments, based on how easily people can avoid certain things."
The results reveal disgust was higher among the participants with access to fresh water and the ability to buy food, which the scientists defined as "rapid market integration".
Feelings of revulsion were lower in the individuals who relied more heavily on hunting and farming.
So-called "pathogen disgust sensitivity" (PDS) appears to therefore "respond to the local cost/benefit context of avoidance"; in other words the pros and cons of eating certain foods.
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The results also suggest "weaker associations between PDS and measures of macroparasite infection".
"Having a high level of disgust sensitivity protected against some kinds of infections, but not necessarily against others like parasitic worm infections that are common in this area," said Dr Blackwell.
"It makes sense because their eggs and larvae are transmitted through the soil, and when you make your living hunting and growing your own food, soils are hard to avoid."
The scientists concluded their results "highlight the importance of evolved psychological mechanisms in human health outcomes".
Speaking amid the coronavirus outbreak, Dr Blackwell added: "Disgust doesn’t protect us very well against pandemics like COVID-19 [the disease caused by the infection] in part, because a lot of people are asymptomatic.
"We don't get any cues of what to avoid. You can't see it."
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