On a fundamental level, the body is put together in very simple ways. All the body’s incredible sophistication—its intricate neural circuits, circulatory, respiratory, and digestive plumbing, and hormonal signaling—is the result of increasingly complex layers of function added on top of basic logics.

And one of the most basic is stress.

Stress is the body being put into a position to face something that (correctly or incorrectly) it perceives it must cope with. Before the word stress was used to describe a physiological process, it was actually used in materials engineering to describe how different things interacted with their environment. The definition of stress in this primordial sense is “a force acting on a resistance.” Some examples:

The weight of a car resting on its suspension system.

A tree straining against a hurricane.

A football player attempting to break a tackle.

Stress by itself is not a bad thing. Just being awake equals more stress than being asleep. Walking equals more stress than sitting down. Any bit of movement or thinking that we do means more stress than not doing it.

A stressed state is different. This is a state is one where the level of stress has risen to a point that the body is putting so much effort into coping with something—it’s putting so much force on a particular resistance (or resistance on a particular force)—that it has to stop doing all kinds of things that it needs in order to stay alive over the long-term.

Pulling an all-nighter, for example.

If you stayed awake studying or working, and you did that long enough or often enough, you’d find yourself getting sicker. This is not because your body got too tired (which it probably did) but more specifically because it didn’t get the chance to recover. Why? Because pulling an all-nighter is far enough out of the body’s functional paradigm—a fancy way of saying “what it does well”—that it had to stop doing all the menial duties and basic upkeep that let it recover well.

Understanding what a stressed state is (and isn’t) is a matter of common sense: even though you can’t stay awake forever just lying in bed, being awake for 16 hours out of the day isn’t a “stressed state.” Why? Because the body is supposed to stay awake (more or less) for 16 hours, and sleep (more or less) for 8. So that pattern of activity allows it to keep recovering at the proper rate, maximizing its ability to stay alive.

So by that logic, you can begin to see how things that could solidly count as stressed states if taken by themselves can be OK when put into the right context. Staying awake for the right amount of time is just OK. But try to stay awake for a week and you’ll go nuts—or at least we can say that your job performance will drop rather dramatically.

You screw with that natural pattern of activity and then you go into a stressed state.

Let’s look at more sporty stuff: sprints. There is simply no way that the body can sustain a sprint for any period of time that even looks like long-term. Just 10 seconds of sustained maximal intensity effort creates a huge amount of anaerobic debt. Attempting to sustain such an effort for a significant period of time would pose an existential challenge to the body’s integrity.

A sprint requires complete shutdown of the processes that keep the body alive in the long-term. Recycling of cerebrospinal fluid, muscle repair, digestion, replacement of red and white blood cells, and sometimes even breathing—it all stops. That’s perfectly OK, or course, because a sprint is expected to stop within 10-20 seconds, and all those processes have a chance to restart again.

But if they don’t—if, for better or worse, the body insistently perceives that the intensity, frequency, or volume of training and racing is threatening to its physical integrity—then it never gets the chance to rest and recover.

All the touted health benefits of HIT—the development of more muscle and bone mass, stimulation of mitochondria, etc.—never get to happen. All that development is a response to the massive tissue breakdown that occurs in high-intensity training. This means that it comes after. If the body perceives that the period of high demand keeps going, it’s going to keep waiting for the right time to build itself back up.

That’s just fine if you train conservatively (read: infrequently enough that your body can rest, and recover, and grow from whatever training it is that you do). But just because a certain kind of training has theoretical benefits doesn’t mean your body can reap those benefits under all conditions.

If the acute stress doesn’t wane, all those critical recovery processes simply won’t restart (or won’t be working well enough to really make a difference—take your pick).

The point is that this is the case with all stressed states. Regardless of why it happens, whenever your body perceives that there is some kind of present threat, a bunch of critical processes are going to stop. (And if some of them are designed to keep other critical processes in check, they’re going to go awry.)

So, if you’re stressed out, and you get sick, it’s far from “just stress.” What it means is that your immune system either shut down (and you got an infection) or it went nuts (and you got a cold) because your body shut down a bunch of systems in order to focus on a clear and present threat to its existence (real or perceived).

If this keeps going for a significant amount of time, your body’s going to start coming apart at the seams.