Have you ever gotten sick during one of the busiest, most stressful periods of your life? You might think it’s terrible luck that you don’t feel well when you can least afford to. But maybe it is not a cruel coincidence. Maybe being busy and frazzled is exact reason you’re under the weather.

When you are stressed, your hypothalamic-pituitary-adrenal axis, or HPA axis, switches on. Your brain sends signals that lead to the release of cortisol and adrenaline. In short bursts, this is helpful. It gives you energy and sharpens your focus. This hormone surge keeps you moving when you need to perform. However, when stress becomes chronic, that system does not fully turn off.

Your level of the stress hormone cortisol stays elevated, your sympathetic nervous system dominates and you live in a low-grade fight-or-flight state. Over time, these higher hormone levels cause the immune system to become dysregulated. Research has shown that chronic stress weakens certain immune defenses while also increasing inflammation.

While you are busy “powering through,” your body is reallocating resources. It is prioritizing survival over long-term repair, which means immune function takes a hit. When the stressor ends—when the big exam is over or when you’ve tackled a problem that’s been on your mind—your body finally exhales. Stress hormones shift and your immune system recalibrates. This is often when symptoms of a cold or other illness show up—not because your luck suddenly ran out, but because your system was running on overdrive for too long.

Chronic stress also changes the brain itself. Prolonged cortisol exposure can alter areas of the brain that help regulate memory and emotion. The burnout you might feel after a stressful event is not just emotional exhaustion, but also biological dysregulation.

Your stress is not “just in your head.” It moves through your hormones, neurons and immune cells. Sometimes getting sick at the worst possible time is not just bad luck. It is the consequence of a body that has been “holding on” for too long.