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When you’re sick you just want to be left alone. Sometimes that’s because you physically can’t move, but other times, it might have more to do with the way your immune system is connected to your brain.

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You know that feeling when you’re sick, where you don't want leave the house, you don't want to talk to anyone, and you kinda just want to be left alone to wallow in your own misery and snot?

Sometimes that's because you're so sick that you literally can't move without risking throwing up or whatever. But other times, it might have more to do with the way your immune system is connected to your brain — including how it affects your social behavior.

A lot of the research on this is really recent, and there’s still plenty we don’t know about that connection. But psychologists and doctors are starting to realize that some immune responses can make you more or less social, sometimes even without symptoms. At first glance, the brain and immune system can seem pretty separate.

Your brain is in its own little compartment, separated from almost everything else by what’s known as the blood-brain barrier. That’s exactly what it sounds like: a literal barrier, in the form of a membrane that keeps your brain safe from whatever’s circulating in your blood. It's really important to have, because it stops almost all germs from infecting your brain.

Meanwhile, your immune system does its thing in the blood running through the rest of your body, trying to protect you from disease. But your brain needs to know what’s going on with your immune system, like if you have a spike in white blood cells to fight off an infection. So there is at least one major connection between them: the vagus nerve, which connects your brain to the parts of your body where a lot of your immune responses happen, like your gut or lymph nodes.

The vagus nerve can detect compounds called cytokines, which are released by your immune system when you're fighting an illness. And we know that your immune system affects your behavior, and that the vagus nerve is an important part of that connection, because of what happens when you cut it. Mice and rats with severed vagus nerves don't show that sickness response where you just want to sit at home by yourself.

They go hang out and party just as much as healthy mice, and they eat just as much too. This normal sickness response — hiding at home, not going out to play with your little mouse friends — is probably an evolutionary adaptation. If you spend a little bit of time being antisocial, you're probably not going to spread whatever germs you're carrying, which is great for society as a whole.

And it helps you on an individual level, too: if you're a little less active, your body can put more resources into fighting the infection. But scientists are starting to learn that the situation is probably more complex than that. For example, one 2010 study published in the Annals of Epidemiology found that immune reactions might be related to an increase in social activity.

Researchers tracked 36 people before and after getting a flu vaccine, and they found the subjects interacted with more people in larger groups right after they got the shot than right before. So, they were spending more time with more people while their bodies were building up an immune response. Now, it's worth noting that this was just a small first study, and it didn't have a control group of people who, say, got a placebo shot.

But the researchers controlled for some obvious problems, like what day they got the shot. It wasn't like everyone got them Friday morning just before going out to party all weekend. The difference between this study and just looking at what people do when they’re actually sick is that in this case, the subjects’ immune systems were more active, but they didn’t have symptoms.

And the results were the opposite of what people do when they’re sick: they were more social when their immune systems were more active. But based on what we’ve learned from later research, being more social when your immune system is stronger might actually make sense. In a study of 121 people published in the journal Psychoneuroendocrinology in 2015—I’m telling you those people named that journal just to mess with us—researchers found that people who were more extroverted and social tended to express more of the genes known to increase the immune response.

In other words, the more outgoing people seemed to have stronger immune systems, which might be another evolutionary adaptation — it could be helpful for them to explore more, take more risks, and meet more people. Again, this research is all really new, and without more studies we can’t know for sure that this is what’s happening. But learning more about the connection between the immune system and social behavior could lead to new treatments for clinical conditions that are related to those behaviors.

For example, we know that some people get more severe symptoms of depression when their cytokines spike in their blood — like when they have a strong immune response. And a similar effect has been found in children with autism: increases of some cytokines in the blood can be accompanied by stronger symptoms. The researchers are starting to investigate ways to use the immune system to affect social behavior on purpose.

In research published in Nature just last year, neuroscientists studied mice that were raised with impaired immune function. Compared to healthy mice, who are much more interested in hanging out with a new mouse than a new inanimate object, the impaired mice were kind of equally interested in both. It was like they saw the other mouse as just another thing.

But, when the researchers injected immune cells called lymphocytes from healthy mice into the impaired mice, that difference went away. We’re still a long way from being able to use the immune system to treat clinical symptoms, but with more research, psychologists hope we might someday be able to. And maybe all this will give you a little hope too, if you're lying on the couch next to a bucket.

You'll probably be interested in hanging out with people again soon — you just have to wait for your immune system to do its job. In the meantime, while you’re sick is a great time to learn weird things about your mind, you can check out our video about whether you can use electricity to supercharge your brain. And if you want to keep getting smarter with us, you can always just go to and subscribe.