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Does the Flu Trick People Into Being Sociable?

There's evidence for a possible behavioral-control component of influenza.

Solomiya Malovana/Shutterstock
Source: Solomiya Malovana/Shutterstock

Once you start thinking about things from an evolutionary perspective, you see the world differently (see Geher, 2014). Today, my daughter is home sick with the flu. Poor kid. As a dad, I feel bad for her. But as an evolutionist, I’m intrigued…

I got thinking about a finding I’d learned about from biological anthropologist Chris Reiber, and wondered how this finding plays out in my own little world. In short, Reiber and her colleagues (2010) explored a very straightforward evolutionary hypothesis regarding the flu. As the flu, like many viruses, travels from one host body into another, it would make sense that the flu virus might actually manipulate people’s nervous systems in a way that increases exposure to a relatively large number of people during the communicable stage of the illness. In other words, perhaps the flu virus makes people unwittingly seek out a relatively high number of social interactions as an evolved mechanism for the virus to get itself into a high number of human bodies.

Sounds like science-fiction, I know. But hear me out...

Do Parasites Affect Behavior?

The answer to this question is straightforward: Yes. Many instances of parasites affecting behavior have been documented (see Moore, 2002). Perhaps the best-known instance of this pertains to Toxoplasma gondii, which travels from cats into other mammals, including mice. And humans, often with adverse consequences.

This parasite has a very interesting effect on the behavior of mice. When evidence of a cat is present, a typical mouse makes itself scarce. But a mouse infected with Toxoplasma gondii does anything but become scarce when a cat is nearby. In fact, an infected mouse shows a strong tendency to go right out into the open — easy prey for the furry killing machines. This parasite must make it into the body of a cat to advance its life course, so it makes good sense that these parasites evolved the capacity to manipulate the nervous systems of mice in such a way.

A Study of the Flu/Sociality Connection

Using similar reasoning, Reiber et al. (2010) came up with a very straightforward, evolution-based prediction regarding the flu. The more human bodies that the virus finds itself in, the more that it replicates. And from an evolutionary perspective, anything that facilitates replication should be considered as something that would have been naturally selected across evolutionary time. In short, it makes sense that the flu might manipulate people to make them interact with a higher proportion of people than usual during the communicable stage.

Now you might be thinking that we all know very well that when someone has the flu, he or she just wants to be alone in a room with a blanket, an iPhone, and Netflix. So what gives?

Reiber et al. (2010) took this point into account via the fact that for the first 24 to 48 hours after one is exposed to the virus, he or she tends to be asymptomatic while being, concurrently, communicable. In other words, when you first have the flu, you don’t even know it — but you are capable of giving it to others. This may actually be part of the evolutionary wiring of the virus.

In their study, which was published in the Annals of Epidemiology, the researchers collected data from 36 adults, all of whom received a flu shot in the fall. Studying individuals who had the flu shot was a diabolical way to test this hypothesis, because the flu shot works by providing individuals with a small but significant dose of the virus. Thus, the researchers were able to study the effect of being infected with the virus on behavior in a controlled manner. And they knew exactly when each individual was provided with the shot, so they could carefully examine the social behaviors that immediately followed.

The researchers asked each participant to document the number of individuals that he or she interacted with for the two days prior to obtaining the vaccine and then for two days immediately after receiving it. Well, wouldn’t you know: The number of people with whom the participants interacted during the post-vaccine period (an average of 99) was nearly double the number they had interacted with during the pre-vaccine period (an average of 54). These findings were (for the stats nerds out there) statistically significant.

Bottom Line

Sure, this was but one study, and it used a relatively small sample — and the specific mechanisms by which the flu might influence behavior were not addressed. But once you start thinking like an evolutionist, the idea of the flu manipulating human social behavior to increase social interactions during the post-infection period makes a lot of sense.

All this said, this flu season, make sure to cover your mouth when you cough, wash your hands a lot, and try not to increase your frequency of social interactions if you think you’ve been infected.


Geher, G. (2014). Evolutionary Psychology 101. New York: Springer.

Moore, J. (2002). Parasites and the Behavior of Animals. New York: Oxford University Press.

Reiber, C., E.C. Shattuck, S. Fiore, V. David, P. van Goozen, and J. Moore. 2010. Change in human social behavior in response to a common vaccine. Annals of Epidemiology 20(10): 729-733. DOI: 10.1016/j.annepidem.2010.06.014