Macaques in groups that lived through a retrovirus outbreak took food from an apparently contaminated surface in 47% of trials. Monkeys from an unaffected group did so 79% of the time.

Disgust is strange for an emotion. It feels immediate and physical, yet what triggers it varies between people and cultures, and it sharpens after an epidemic. Where it comes from, and how much of it is learned, has been difficult to study in humans because almost everyone already knows that germs cause disease.

Researchers at the University of California, San Diego have found a way around that. Writing in Royal Society Open Science, they report that macaques whose groups lived through a lethal virus outbreak became markedly more cautious about food that looked contaminated, years afterwards.

“We can show now, in non-human primates, that this is the same for them,” said Federico Rossano, an associate professor in the Department of Cognitive Science at UC San Diego’s School of Social Sciences and the study’s senior author. “Even with no knowledge of how disease is transmitted, being exposed to others getting sick and dying changes the likelihood of engaging in behaviors that potentially could lead to some form of sickness.”

An outbreak nobody planned

The study took advantage of a natural experiment at a primate sanctuary in southern Texas, home to Japanese macaques descended from monkeys brought from Japan in 1972.

In 2020, staff confirmed simian retrovirus Type II in some of the sanctuary’s Japanese macaque groups. The virus spread mainly through bites and scratches, and it caused severe illness and deaths. The sanctuary separated the groups and introduced testing and quarantine.

That left three groups that had experienced the outbreak and one that had not, a comparison no researcher could ethically have created.

Food on a dirty surface

The test was simple. The team offered 55 macaques a choice between food on a clean wooden surface and food on a surface marked with sealed, odourless faeces, so that the cue was visual rather than a smell.

Monkeys from the outbreak-exposed groups took the food from the apparently contaminated surface in 47 per cent of trials. Monkeys from the group that had not experienced the outbreak did so in 79 per cent.

Crucially, monkeys from the outbreak groups behaved similarly whether or not they had themselves been infected. The change tracked having lived through the event, not having been ill.

That distinction matters for interpretation. An animal that got sick might have learned from its own illness. An animal that stayed well but watched others sicken and die has learned from something else, which is closer to the social transmission of caution.

The researchers also ran trials in which a brightly coloured sponge replaced the clean surface, to check that the monkeys were avoiding contamination rather than simply shunning anything unfamiliar.

That control addresses a standard objection to experiments of this kind. An animal that refuses food beside an unfamiliar object may be reacting to novelty rather than to contamination, and a result built on that confusion would say nothing about disease avoidance. Sealing the faeces so they could not be smelled serves the same purpose from the other direction, isolating the visual cue.

What early life does

A second study, of 54 macaques from seven species with different upbringings, found another pattern. Former pets were significantly more likely to eat the contaminated food than macaques born at the sanctuary and raised by their mothers.

“One possibility is that these monkeys didn’t have the opportunity to learn typical avoidance behaviors from their mothers and other macaques early in life,” said Stephan Kaufhold, a doctoral researcher in cognitive science who led the study. “Even after living with other monkeys for several years, that early experience may still shape how they respond to potential sources of disease.”

That is an argument for avoidance being partly taught. If aversion to contamination were purely innate, a monkey’s rearing should not matter much. If it is learned by watching others, an infant raised among humans would miss the lessons, and the deficit could persist into adulthood.

Not absolute, but negotiable

Earlier work suggests the behaviour is a calculation rather than a reflex. Wild Japanese macaques would eat a favourite treat from a faeces-covered surface, while refusing less-valued food in the same situation.

That is worth noting against the figures here. Even the wary groups still took the contaminated food nearly half the time, which indicates reluctance rather than prohibition. The monkeys appear to weigh how much they want the food against how risky it looks, and the outbreak shifted the weighting.

Humans do something similar. The same food dropped on a clean kitchen floor and on a pavement draws different reactions, and hunger changes both.

The timing of the Texas outbreak, in 2020, places this work alongside a body of human research from the same period showing that people’s disgust sensitivity and willingness to tolerate contamination shifted during the COVID-19 pandemic. What the macaques add is a case where the subjects had no theory of germs at all.

The origins of disgust

“Groups have histories, and those histories show up in behavior,” Kaufhold said. “Living through the disease outbreak appears to have made those monkey groups more cautious around food that looks contaminated.”

The authors are careful about terminology. They did not measure disgust, which is an internal state they cannot access in an animal. They measured avoidance, which is a behaviour.

The relevance to human psychology lies in the mechanism. Disgust is widely understood as a behavioural immune system, a set of responses that keep people away from pathogens before the actual immune system has to act. The question has been how much of that system is hardwired. These results suggest that in primates at least, the triggers are tuned by experience, both an individual’s early life and a group’s collective history.

The researchers suggest that heightened avoidance might be learned socially and persist in a group, and that future work could test whether newly introduced animals pick up the behaviour from established ones. If they do, the sanctuary’s monkeys would be carrying a cultural memory of an epidemic that most of them never caught.

The release does not state the study’s funding sources.

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Notes

1. Kaufhold, S., Rossano, F., et al. “Early-life experience and disease exposure affect pathogen avoidance behaviour in macaques.” Royal Society Open Science, 20 October 2026.
2. University of California – San Diego. “Like humans, some primates become cautious of germs in the wake of a viral outbreak.” EurekAlert!, 20 October 2026. https://www.eurekalert.org/news-releases/1147821.