As the bell rings and you put down your exam paper, you breathe out a sigh of relief.

Finals are done, and in a few days you’ll be sitting on a beach somewhere. But when you actually get there.... you’re suddenly sick.

If this has ever been you, you’re not alone. Instead of getting sick during a stressful situation, like a week of tests or a big project at the office, it’s super common for people to get sick afterwards. Some of it could have to do with the bad habits you pick up during project season.

But a lot of the reason is because of the way your immune system responds to stress. If you were someone who handled finals by cramming at 3AM, fueled by beef jerky and energy drinks, you probably weren’t that surprised when you came down with a cold. Bad habits, especially a lack of sleep, can make you more likely to get sick.

But, even if you were to treat your body like a temple, the stress alone is enough to make you more susceptible to illness. The first thing your body does when it’s stressed is activate its fight or flight response, which you’ve probably felt before. It’s best known for that classic fast heart rate and breathing, but this response has an immediate effect on the immune system as well.

It affects a bunch of hormones, but the big player is an increase in cortisol, the stress hormone. That bump in cortisol helps signal for a shift from a more generalized immunity to a response that’s aimed at a specific pathogen. See, normally, when any invader gets in one of your cells, it’s labeled with a marker that identifies it as not you.

Then, your body tries to attack it right away. This process is called cellular or cell-mediated immunity, but it’s hindered during stress. Instead, your body signals for an increase in something called humoral immunity.

This is the side of your immune system that makes proteins to fight specific pathogens. But instead of kicking in right away, it takes a few days, since your body has to make enough of those proteins. Researchers still aren’t totally sure why this transition happens, but it might be a way for your body to limit that all-over sick feeling while it tries to survive the stressful event.

So, while you’re busy memorizing formulas, your immune system is sitting quietly in the corner, building proteins and leaving you alone. Then, when finals are over, your cortisol levels drop back to normal, and your immune system relaxes as well. The problem is, it might relax too much.

Many of the hormones that signaled for that increased immune activity are actually suppressed when the stress is over. And without messengers signaling for your cells to attack, your body won’t waste energy to do so. That means that, after finals, you’re suddenly an easy target for a pathogen to call home.

And, if you’re spending your break on a crowded airplane or a packed beach or if you actually picked up a pathogen during finals that’s bad news for you. Get the tissues ready. And if you’re already down for the count and ready to binge on videos until you’re healthy again, we recommend one of our sister channels, Healthcare Triage.

On this channel, Dr. Aaron Carroll explains healthcare policy, medical research, and answers a lot of other questions you may have about medicine, health, and healthcare. So check it out at YouTube.com/HealthcareTriage and I hope you stay healthy!