Some people are so depressed that they can't function or can function very little. That would be an illness. There are different degrees of depression.
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Also, you seem to equate depression with pessimism. I don't think the two are equivalent. One could be a pessimist and not be depressed. Pessimism is really just another term for realism. Schopenhauer was a pessimist. But he still liked wine, women and song.