Influenza comes like a bolt from the blue, attacking its victims with disconcerting suddenness. To be sure, breakdowns may appear, in many cases at least, to come in a fell swoop; but what seems so abrupt and unlooked for is usually the climax to a long-continued process of undermining, like the collapse of a house, which has succumbed to the ravages of time. Yet the events which have led up to it may have been spread over a large number of years. [Sidenote: The nature of a breakdown.] Occasionally we hear of someone who has been disabled all of a sudden by some definite form of ailment, paralysis, cancer or heart disease, it may be. Such cases are, however, the exception, and they are not the breakdowns with which we are now concerned. In the great majority of instances “breaking down” is the final stage of a long process of “running down.” There is as much difference between the two classes of cases as between an engine which has come to a stop because a wheel has come off or a connecting-rod broken, and one that has become useless owing to neglect or prolonged wear and tear. The period of running down may last for months or years, and it is characterised by various symptoms, physical and nervous. It is the former which are at the root of the matter, but the others predominate more and more, until, when the final breakdown occurs, they overwhelm the bodily symptoms altogether. On this account it is usually designated by terms expressing this nervous element: nervous exhaustion or debility, neurasthenia or simply nerves. Yet all these are only different phases or stages of the same complaint. What, then, is the nature of this complaint? It is one that has suffered from much misapprehension, chiefly through the use of the term “nervous exhaustion.” This phrase has given rise to an impression in the lay mind that there is a limit to the nervous force with which human beings are endowed, as though each one started with a certain quantity which must come to an end sooner or later. This idea is a fallacy, for nervous energy is in process of being manufactured every hour we live. And Nature stores up out of this supply a reserve from which we may draw in any emergency that may demand a special output. This reserve fund is constantly varying. It is replenished during the hours of sleep, it is called upon during the period of wakefulness. Sometimes an extra call has to be made upon it. A woman may have her night’s rest broken, or she may even lose her sleep altogether for several nights in succession. Or a man may have a sudden stress of work which cannot be avoided. Then the reserve may be depleted, but that does not constitute a breakdown. If care is taken to ensure sufficient rest afterwards, the surplus is regained. It is only when a constant drain is put upon it that serious damage results. [Sidenote: The two types.]