Depression is the leading cause of disability in the world.

In the United States,

close to 10% of adults struggle with depression.

But because it's a mental illness,

it can be a lot harder to understand than, say, high cholesterol.

One major source of confusion is the difference between having depression

and just feeling depressed.

Almost everyone feels down from time to time.

Getting a bad grade,

losing a job,

having an argument,

even a rainy day can bring on feelings of sadness.

Sometimes there's no trigger at all.

It just pops up out of the blue.

Then circumstances change,

and those sad feelings disappear.

Clinical depression is different.

It's a medical disorder,

and it won't go away just because you want it to.

It lingers for at least two consecutive weeks,

and significantly interferes with one's ability to work,

play,

or love.

Depression can have a lot of different symptoms:

a low mood,

loss of interest in things you'd normally enjoy,

changes in appetite,

feeling worthless or excessively guilty,

sleeping either too much or too little,

poor concentration,

restlessness or slowness,

loss of energy,

or recurrent thoughts of suicide.

If you have at least five of those symptoms,

according to psychiatric guidelines,

you qualify for a diagnosis of depression.

And it's not just behavioral symptoms.

Depression has physical manifestations inside the brain.

First of all,