A mental illness well known, but not fully understood. Ironic, considering how many times it is misdiagnosed. Now, as a writer, the only way I know how to explain such a concept is creatively.
Imagine yourself in a sink hole, unable to escape. At first, you try to claw your way out towards the light; only to realize the effort is in vain. The evidence lies in your bleeding fingers and claw marks in the sand. Slowly, the darkness swallows your whole being; giving yourself over to its embrace. Hours pass as your need for food, drink, socializing.. anything fades away. All you will do is stare blankly into the abyss, questioning why you should even exist. There is no finding a way out. If you are lucky, a friend will throw a line down to raise you out of the hole. Whether you accept that offer is a different story. They will have to fight for you to find the hope to crawl to them; pull you back from the abyss. Otherwise you are at the mercy of the Earth, if you have the right herbs. These may just snap you out of it or the wrong amount will make things worse.
Be mindful of your fellow humans if you see such suffering, you could be the line that helps them out of the hole.