Each of these "stages" in the evolution of "disease" is marked by its own characteristic developments, the sequences of events following in orderly fashion throughout. In other words the phenomena of biogony are as lawful and orderly in their progress, development and decline as any other series of phenomena in nature.

The course of biogony may be short and severe--acute; or long and mild--chronic. Acute and chronic are relative terms, no positive line of demarkation dividing the two classes of biogonies. Some biogonies present a rapid succession of symptoms at first, and after a few days become less active and, finally, are slow and much protracted. The three stages of this course are denominated acute, subacute and chronic.

Acute biogonies are subject to rhythmic rises, (exacerbations) and falls (declines) of symptoms. All movements in nature are intermittent and not continuous. The tide not only rises; it also falls. All advance is an advance and a recession and another advance and another recession, the advances preponderating over the recessions. A period of activity is followed by a period of repose. Physiological actions are rhythmic--there is periodicity or regularity in physiological rhythm.

In biogonies, the interruption of the periodic rhythm ingrained in the constitution of the creature is not so great but that intermittency in action is apparent. There is a rise and fall of the tide of defensive action as the body rests or renews its intensity of curative effort. The various organs struggle to improve their condition, partially succeed, and then demand a period of comparative rest. The natural activities of the body allow periods of repose or of comparative repose--the alternate periods of action and repose differ in frequency in the several organs, providing the longest and most general quietude at night, gradually exalting and then gradually abating the general intensities of the functions with the advance and decline of the day.

The periodic rise and fall in activity in biogony follows the regular physiologic rhythm of health. The general efforts of the organism tend to be greater in the middle and later portions of the day, while a measure of quietude is secured through the middle and later portions of the night. The duration of the exacerbation and remission varies--the former sometimes occupying nearly the whole twenty-four hours; but the period of intense effort growing shorter as the organism approaches the healthy condition, the remission growing more distinct and prolonged at the same time. When there is feebleness of the vital powers these ebbs and flows of action will be less marked; while, they are also very indistinct in most chronic affections.

The period of intense effort is called exacerbation, or the paroxysm, while the period of abatement is a remission or an intermission. Exacerbations represent dynamic actions of the body against pathogen and are periodic because they are of vital origin and the living organs must have rest, which they get during the period of remission.

As pointed out above, in most cases the exacerbations begin towards noon, gradually increase in severity till early evening, and distinctly decline about midnight--the remission occurring in the later hours of the night and early morning. In a few cases the symptoms manifest no tendency to abate, from the beginning of the trouble till a decided change toward recovery or death occurs. These are the so-called continued types. Exacerbations and remissions are seen in all chronic conditions, but are commonly less distinct and the exacerbations are farther apart.

Most acute "diseases," such as smallpox, measles, diphtheria, etc., present certain conditions and symptoms succeeding each other with such marked regularity and in quite uniform periods of time, these developments being modified to some extent by the care the patient receives (these maladies terminating in health, unless treatment or other factors have resulted in death, after a certain number of days), that these maladies are said to be self-limited. Other acute "diseases," yellow fever, for instance, pass through a regular series of changes, but are not so definite in time.

The clinical history of most acute "diseases" presents a succession of changing conditions marked by changes in symptoms. Three periods have been defined.

1. Incubation absurdly called invasion, embraces the period when the causes of pathology are accumulating in the body. During this stage the exacerbations and remissions occur. Eruptions, exudations, effusions, etc., also occur during this stage.

Acute "disease" commonly succeeds a series of prodromal symptoms--a sense of weariness or lassitude, with yawning and stretching; aching, or even a feeling of bruised soreness in the muscles; pains in the limbs and back, mingled perhaps with a sense of chilliness; probably headache, dizziness, loss of appetite quite constantly, and sometimes nausea. Coetaneous with these, there is a vague sense of uneasiness or discomfort; mental powers are depressed and sleep is disturbed or unrefreshing. There is neither fever nor increased circulation during this developing or premonitory stage.

This stage may be quite trifling and last but a few hours, or strongly pronounced and last for two or three days, or less distinct yet distinct and continue for many days with a gradual increase in intensity. The loss of strength and mental depression are greater than can be accounted for by the slightly diminished function present.

2. Reaction is that period during which the vital powers exert intense resistive efforts to overcome the causes of pathology. This is the fever stage, when the lungs, heart and general circulation, the nerves and other organs have entered vigorously into the work of resisting pathogen and are trying to preserve the vital domain.

Fever and the other symptoms accompanying it appear to result from a shock to the nervous system from which the organism reacts with increased action. A healthy nervous system will not so quickly fall victim to pathogenic causes and will throw these off more quickly than the enfeebled and languid nervous system. In the first condition the period of incubation will be brief, and circulatory reaction prompt and vigorous; in the second condition the incubation period will be prolonged, reaction slow, vacillating and feeble. A vigorous system will present more violent symptoms and recover quickly; a depressed system will present less violent symptoms, the "disease" will be much prolonged, and there is more likely to be a profound exhaustion.

Termination is the end of the intense vital struggle. The process may terminate in recovery, in a dynamic biogony, or in death. In most cases the organism is able to throw off the impairing causes and restore health. In quite a few cases the body is forced to delay the action of these causes, as in passive biogony, and perhaps finally overcome them altogether. When the sum of the vital actions in biogony is fully able to overcome pathogen and repair damages, the case terminates in health. Termination in death may be gradual, the vital powers failing by slow gradations till their activities cease; or it may be sudden, occurring in a few hours, or almost momentarily. Death represents a complete failure of biogony and a triumph of pathogen. Chronic "disease" is a partial success of biogony and the persistence of pathogen.