When you're feeling under the weather, there's a reason you'd rather stay in and watch a movie alone than visit a theater with friends. And it isn't necessarily due to your nagging cough, or because you'd be forced to put on pants: As the SciShow's Hank Green explains in the video below, our immune systems—which are connected to our brains—may affect our social behavior.

The brain is linked to the immune system through the vagus nerve, a vast network of fibers that connects to parts of our body like the gut or lymph nodes. This nerve can detect compounds called cytokines, which control various inflammatory responses. Your immune system releases cytokines when you're battling an illness, and your brain can sense them through this complex communication chain, resulting in you becoming more withdrawn.

Learn more about the science of why you feel like flying solo on sick days—and on the flip side, how some immune responses can make some people more social—by watching the video below.