Most of those who read this piece know that COVID-19 is caused by a virus (the awkwardly named SARS-CoV-2 virus, to be precise). And we know that the virus’s spread can be inhibited via handwashing, not touching our faces, and avoiding crowded areas. We owe this knowledge to the germ theory of disease, which was developed only 150 years ago, and the heroic efforts of contemporary virologists. How did humans avoid pathogens for the other 99.9% of our time on earth, when no one knew of viruses and bacteria? And can the answer to this question offer some clues to understanding why diseases like COVID-19 are so problematic now?

The Behavioral Immune System

Throughout our evolutionary history, the microbes that cause disease have not been randomly distributed across the environments we inhabit. They were (and remain) more likely to be housed in rotting meat than a tree, in human feces than human hair, and in someone whose skin is covered with lesions than someone whose skin is clear. Given such regularities, our sensory systems evolved a sensitivity to the colors, textures, and smells that correlated with pathogen presence. That is, we evolved to treat some aspects of our environment as cues to pathogens. Such cues gave our ancestors a fighting chance to steer clear of the infectious microbes that they didn’t even know existed.

Neutralizing pathogens requires more than detection; it also requires avoidance. The emotion disgust (and, specifically, pathogen disgust) serves this function (Tybur et al., 2013). But it does not engender just any kind of avoidance. Contrast disgust with the fear evoked by things like crocodiles and cobras, which can traverse a meter in the blink of an eye. Viruses and bacteria are different; they have no muscles to propel themselves, and they largely get inside of us when we reach out and touch something. Hence, neutralizing pathogens doesn’t require the rapid flight engendered by fear; it requires a revulsion that motivates us to avoid contact.

A bevy of evidence suggests a match between disgust and pathogen threats. One study of 40,000 participants across six continents and nine cultural regions found that participants were more disgusted by pictures of infection threats than similar images posing no infection threat (Curtis et al., 2004). Surveys of the 25 deadliest infectious diseases across human history revealed that all 25 produce visible symptoms, most of which elicit disgust (Oaten et a., 2011). And hunter-gatherer populations without Western scientific educations have a sense of contamination – that is, they won’t eat something that has contacted another object that elicits disgust (Apicella et al., 2018).

Psychologists often refer to our pathogen-detection and avoidance adaptations as a behavioral immune system, which they have recently aimed to better understand (Ackerman et al., 2018; Murray & Schaller, 2016; Tybur & Lieberman, 2016). This behavioral immune system is a marvel of natural engineering – it evolved to detect and avoid the pathogens that scientists would not identify as the cause of infection until the 19th century. And it has consequences for myriad phenomena ranging from social exclusion to food choice to sexuality.

Bypassing the Behavioral Immune System

If we’ve evolved an ability to detect and avoid pathogens, why do we still get sick? There are a few answers to this question. For one, the behavioral immune system relaxes avoidance motivations to accommodate behaviors that require some infection risk. Think about how parents scarcely bat an eye when cleaning their baby’s vomit, or how lovers don’t recoil at the thought of swapping spit. Even if such vomit and saliva are no less likely to carry pathogens than another baby’s vomit or another person’s saliva, a disgust response would compromise investment in our helpless offspring and in reproducing.

Further, while the behavioral immune system is far better than nothing, it is imperfect. Features that trigger its alarms sometimes do not correspond with pathogens, like the stench of the durian fruit. And pathogens sometimes skirt by undetected, like the E. coli that occasionally contaminates leafy vegetables.

The behavioral immune system also does little to neutralize the pathogens transmitted by the blood-sucking arthropods, like mosquitoes, ticks, and tsetse flies. These creatures have evolved to avoid detection for their own interests, and many pathogens have evolved to capitalize on this stealthiness (Zimmer, 2001).

Features of our densely-populated modern environments also allow pathogens to bypass our behavioral immune system defenses. Consider conditions in the Vienna General Hospital in the mid-19th century. For years, approximately 10% of women who gave birth in one of the hospital’s clinics died from puerperal fever (bacterial infection of the female reproductive tract). Ignaz Semmelweis, a key figure in the development of the germ theory of disease, noted that physicians here often performed autopsies before delivering babies. He correctly posited that this sequence resulted in some kind of cadaverous materials being transferred from corpses to patients. Had the women in the maternity ward seen their physicians’ hands entering a corpse before assisting with birth, they likely would have fled in revulsion.

Supplementing the Behavioral Immune System

We do not need to develop innovations to neutralize the infection risks that would arise if people regularly ate feces or licked corpses. The behavioral immune system already steers us away from these types of behaviors. But neutralizing the infections that are destructive, such as COVID-19, requires us to fill the gaps in our behavioral immune system defenses.

We can look to Semmelweis for inspiration. He didn’t just identify the cause of the high mortality rate in this clinic; he also developed a new handwashing protocol after autopsies were performed. Following this new practice, fatalities in the maternal clinic plummeted. His efforts led him to be known both as The Savior of Mothers and The Father of Hand Hygiene. And he provides a textbook case for how cultural innovations can supplement the behavioral immune system.

Semmelweis’s legacy – hand hygiene – is a key weapon in the battle against COVID-19. As are the social distancing measures that were implemented by China in early 2020 and are currently being implemented across the world. Mosquito nets offer another example. They reduce malaria, dengue fever, and yellow fever in tropical regions, and they may soon need to be applied in other parts of the world due to climate change. Reducing infections may also require intervening where the behavioral immune system relaxes by design, such as the lower aversion we feel toward, say, drinking from the same water bottle as a friend relative to drinking from the same water bottle as a stranger.

Of course, innovations are only useful if they are accepted and adopted. The behavioral immune system can assist in encouraging such adoption. Take hand-washing as an example. Just as the mothers in Semmelweis’s hospital would have to be revulsed by indirect contact with a corpse, people now might be disgusted by the scope of indirect contact facilitated by public surfaces. Whereas viruses are an unseen abstraction to most of us, others’ bodily fluids and wastes are not. Facilitating such connections might help enlist the behavioral immune system in our fight against modern pathogens.


Ackerman, J. M., Hill, S. E., & Murray, D. R. (2018). The behavioral immune system: Current concerns and future directions. Social and Personality Psychology Compass, 12, e12371.

Apicella, C. L., Rozin, P., Busch, J. T., Watson-Jones, R. E., & Legare, C. H. (2018). Evidence from hunter-gatherer and subsistence agricultural populations for the universality of contagion sensitivity. Evolution and Human Behavior, 39, 355-363.

Curtis, V., Aunger, R., & Rabie, T. (2004). Evidence that disgust evolved to protect from risk of disease. Proceedings of the Royal Society of London. Series B: Biological Sciences, 271(suppl_4), S131-S133.

Murray, D. R., & Schaller, M. (2016). The behavioral immune system: Implications for social cognition, social interaction, and social influence. Advances in Experimental Social Psychology, 53, 75‒129.

Oaten, M., Stevenson, R. J., & Case, T. I. (2011). Disease avoidance as a functional basis for stigmatization. Philosophical Transactions of the Royal Society B: Biological Sciences, 366, 3433-3452.

Tybur, J. M., & Lieberman, D. (2016). Human pathogen avoidance adaptations. Current Opinion in Psychology, 7, 6-11.

Tybur, J. M., Lieberman, D., Kurzban, R., & DeScioli, P. (2013). Disgust: Evolved function and structure. Psychological Review, 120, 65-84.

Zimmer, C. (2001). Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. Simon and Schuster.