Have you ever wondered why your back aches when you're down with the flu or a cold? Or COVID?

This discomfort, common during many illnesses, is not just a random symptom. It's a result of complex interactions between your immune system and your brain called the "neuroimmune synapse."

When our body detects an infection, our immune system releases molecules including signaling proteins called cytokines. These proteins coordinate our immune system to fight off the infection and talk to our brain and spinal cord to change our behavior and physiology.

This can result in symptoms like fatigue, loss of appetite, fever and increased sensitivity to pain. Classically, we think of this as a beneficial behavioral change to help us conserve energy to fight off the infection. It's why we often feel the need to rest and withdraw from our usual activities when we're sick—and also why we are grumpier than usual.