Depression is the leading causeof 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 understandthan, say, high cholesterol. One major source of confusion isthe difference between having depression and just feeling depressed. Almost everyone feels downfrom time to time. Getting a bad grade, losing a job, having an argument, even a rainy day can bring onfeelings 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 lotof different symptoms: a low mood, loss of interest in thingsyou’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 fiveof those symptoms, according to psychiatric guidelines, you qualify for a diagnosis of depression. And it’s not just behavioral symptoms. Depression has physical manifestationsinside the brain. First of all, there are changes that could be seenwith the naked eye and X-ray vision. These include smaller frontal lobesand hippocampal volumes. On a more microscale, depression is associated with a few things: the abnormal transmission or depletionof certain neurotransmitters, especially serotonin, norepinephrine,and dopamine, blunted circadian rhythms, or specific changes in the REMand slow-wave parts of your sleep cycle, and hormone abnormalities, such as high cortisol and deregulationof thyroid hormones. But neuroscientists still don’t havea complete picture of what causes depression. It seems to have to do with a complexinteraction between genes and environment, but we don’t have a diagnostic tool that can accurately predict whereor when it will show up. And because depression symptomsare intangible, it’s hard to know who might look finebut is actually struggling. According to the National Instituteof Mental Health, it takes the average personsuffering with a mental illness over ten years to ask for help. But there are very effective treatments. Medications and therapy complementeach other to boost brain chemicals. In extreme cases,electroconvulsive therapy, which is like a controlled seizurein the patient’s brain, is also very helpful. Other promising treatments, like transcranial magnetic stimulation, are being investigated, too. So, if you know someone struggling with depression, encourage them, gently, to seek outsome of these options. You might even offer to helpwith specific tasks, like looking up therapists in the area, or making a list of questionsto ask a doctor. To someone with depression, these first steps can seem insurmountable. If they feel guilty or ashamed, point out that depression is a medical condition, just like asthma or diabetes. It’s not a weakness or a personality trait, and they shouldn’t expect themselvesto just get over it anymore than they could will themselvesto get over a broken arm. If you haven’t experienced depression yourself, avoid comparing it to timesyou’ve felt down. Comparing what they’re experiencingto normal, temporary feelings of sadness can make them feel guilty for struggling. Even just talking about depression openly can help. For example, research shows that askingsomeone about suicidal thoughts actually reduces their suicide risk. Open conversations about mental illnesshelp erode stigma and make it easier for people to ask for help. And the more patients seek treatment, the more scientists will learnabout depression, and the better the treatments will get.