There are two broad classes of 'depression'.
Traditional depression can be called melancholia, or endogenous depression - it is rare, very rare - roughly a tenth of one percent prevalence (0.1% of the population) - including the even rarer and more severe 'psychotic depression' with hallucinations and delusions.
Melancholia is severe and debilitating (sufferers typically cannot look after themselves and nearly always require hospitalization of equivalent intensity of supervision); it comes-upon somebody like an illness.
Sufferers from Endogenous Depression have a very high rate of suicide and may dehydrate/ starve themselves to death. It is perhaps the worst kind of suffering of all.
But if the Melancholia/ Endogenous Depression sufferer survives/ is kept alive; after about a year he will probably recover fully (except for the memories) and return to normal; and he probably will not suffer another episode. This recovery can usually be hastened with some tricyclic drugs like imipramine, and electroconvulsive therapy/ electroshock.
So, melancholia a disease which can probably affect anyone, lasts about a year, and after recovery a return to normal.
(SSRI-type drugs such as Prozac are ineffective in Melancholia, or make things worse - so these drugs should never have been called antidepressants. SSRIs aren't antidepressants - SSRIs cannot do what imipramine does. To have claimed otherwise was a dishonest but successful marketing ploy, not a clinical observation.)
The usual 'depression' diagnosed and treated nowadays is more like a personality type than a disease, therefore modern depression is a lifetime thing. Episodes are exacerbations of the usual situation for that person, and 'recovery' means simply a return to the usual suboptimal situation for that person.
The most-commonly diagnosed 'depression' nowadays is therefore nothing like melancholia/ endogenous depression at all - except for the slight similarity that misery/ sadness/ inability to experience happiness may (or may not) be the most prominent symptom.
Depression nowadays gets diagnosed in around ten percent of the population - or sometimes even more. While significantly unpleasant, it is not usually severe - sufferers continue to work, look after families, drive - they continue to eat and drink and attend college.
And the suicide rate in modern depression is not increased - it is the the same as the general population.
Ninety-nine out of a hundred cases of modern depression are part of a mixed bag of diagnoses that can be gathered under names like Neurotic Depression/ Reactive Depression/ Dysthymia - and indeed until the 1990s were usually diagnosed as Anxiety States. The main symptoms are varied and would usually be anxiety, sadness, or mood swings; more rarely low vitality.
The main generalization to be made about modern depression is that it is not like a disease which comes over somebody, it is like a personality type which sometimes gets worse but (because it is the personality) is not the kind of thing which can be cured.
So, melancholia is rare and terrible but can be cured and the sufferer recovers fully' modern 'depression' is common, unpleasant but not terrible, but cannot be cured - symptoms can be suppressed for a while by various types of drugs; but when you stop the drug, or when it loses effect from 'ttolerance', then the personality re-emerges - so the usual suboptimal personality state will return plus added problems from the drug withdrawal.
This means that when taking drugs to treat the usual (Neurotic/ Reactive) depression, there is a strong tendency to get hooked. Because drugs tend to lose their effect ('tolerance') there is a tendency to escalate doses; because the personality is pretty much fixed, there is a tendency to continue treatment for a long time - forever; because all psychoactive drugs produce withdrawal phenomena, then reducing or stopping the drugs will creates additional problems.
In sum, once people with an exacerbation of their usual state of Neurotic Depression (e.g. anxiety, sadness, mood swings, demotivating) have started taking drugs, there is a tendency for this drug taking to become life-long.
This is because the drugs are not curing a disease, but temporarily modifying a personality; and because the body and brain adapt to drugs by becoming dependent on them.
Hence we have a serious, long-term, still-growing epidemic of prescribed-drug dependence among people with Neurotic Depression - with the prescription rates for so-called SSRI-type 'anti-depressants' climbing year upon year, even three decades after the SSRIs were introduced.
There is zero justification for treating Neurotic/ Reactive Depression in order to prevent suicide.The only sensible rationale for drug treatment is to make people feel better; recognizing that treatment will be a temporary suppression of problems not a cure; and any drug treatment will create problems of dependence that will have to be managed.
Modern depression is increasingly being re-labelled as Bipolar Disorder; on the basis of a supposed but false resemblance with Manic Depressive Disorder. True Manic Depressive Disorder was extremely rare - a small fraction of 0.1% - and was diagnosed on the basis of a person having been hospitalized for sustained episodes - often lasting for months - of both melancholia and mania; from which there was complete recovery.
Modern bipolar disorder is diagnosed at a prevalence rate of about 5% (one in twenty) one the basis of self-reports of both high and low moods lasting as little as a few days or weeks and no history of hospitalization; and typical patients never fully recover and seem to be ending up on lifelong treatment with multiple drugs.
The situation developed in a way closely analogous to depression. In other words, modern Bipolar Disorder is being applied to a subgroup of much the same population described above as having 'Neurotic Depression' - that sub-group in which mood swings are prominent. In other words, Old Manic Depressive Disorder was a very rare disease, modern Bipolar Disorder is a relatively common (group of) personality type(s) - being misinterpreted as disease.