Depression is a biologically based disorder that can be treated. And treatment improves more than your mood. It can improve your physical health, too. And, of course, feelings such as sadness are part of life. But it is never normal to be depressed for long periods, usually defined as two weeks or more.
Depression is a treatable, biologically based illness caused by chemical imbalances in the brain.
It appears there may be physical reasons that depression often accompanies other medical illnesses. For example, studies suggest that depression and other diseases, such as heart disease or dementia, may be interrelated. Sometimes, the biological mechanisms of depression can be linked to, or triggered by, the chemical changes brought about by another illness. Other studies suggest that depression may be a risk factor for illness. The good news is that treatment works.