A classification of diseases has not so much value in reference to therapeutics as to the investigation of the character of the disease.

We may divide diseases in reference to the individuality of the patients into

(a.) Diseases belonging to peculiar ages (morbi aetatum), which may be either acute or chronic. Owing to the extreme irritability of the childish organism, it is peculiarly liable to spasmodic sufferings; the reproductive system being principally active in the child, it must be subject to diseases which are principally seated in the lymphatics; the disturbances which occur in the reproductive system, maintained and increased by the want of irritability and by the inactivity of the lymphatics, are characterized by congestion to the brain in the form of epistaxis, meningitis, hydrocephalus, typhoid symptoms of various kinds, scrophulosis, helminthiasis, etc. In a more advanced age, when the vitality of the thoracic organs is developed in a superior degree, congestion of the lungs is a prevalent condition, inducing a corresponding predisposition to pulmonary diseases; in this age the sexual organs develope themselves and the passions connected with that development begin to be felt, and, if satisfied to excess, lead to various diseases peculiar to this second period of life. The smallest number of diseases occurs in the period when the human organism is fully and harmoniously developed. As man advances in age, the abdominal organs are principally affected, hence atony of the intestinal canal, hypochondria, haemorrhoids, gout, etc. are the principal diseases of that period. Old age, when all the moral and physical energies of man are on the decline, is especially predisposed to paralysis of every kind, deafness, blindness, apoplexy, asthma, paralysis of the lungs, affections of the bladder, etc. Diseases affecting the organism during a transition period (which may be said to occur every seventh year,) are of a higher importance on account of the development which the organs undergo during that period.

(b.) Diseases belonging to the different sexes (morbi sexus). The difference which prevails in the character and degree of the irritability, sensibility and reproduction of the female and the male organism and in the physical as well as psychical tendencies of the two sexes, makes each of them liable to peculiar diseases. Suffice it to mention the various nervous diseases to which women are subject; the diseases depending upon the peculiar sensitiveness and irritability of the female temperament; the various diseases affecting the reproductive system of the female organism, such as tuberculosis, carcinoma, scirrhus, etc.

(c.) Diseases belonging to particular classes and trades. Rich people, who are accustomed to rich and luxurious living and spend their life in idleness and ennui, are liable to derangements of the abdominal organs and consecutive diseases, such as gout etc., whereas the poor are affected with diseases resulting from an impoverished reproduction. Tanners are subject to dropsies; type-founders, miners, potters to tabes metallica; tailors and workers in wool to scabies; compositors and printers to oedema of the feet and varicose conditions; chimney-sweeps to gangrene of the genital organs; stone-cutters, hairdressers, millers to pulmonary phthisis; washerwomen to dropsy; literary men who lead a sedentary life, to diseases of the abdominal organs; mariners and fishermen to scurvy, anasarca, etc.

What has been said in the preceding paragraphs, is sufficient to show all the essential points which the physician ought to be informed about in order to obtain a correct knowledge of the origin and course of the disease, and even the internal changes which characterize it; that knowledge being indispensable to a sure and successful treatment.

Diseases may also be classed according to the region over which they spread. We have

(a.) Sporadic (morbi sporadici), or diseases which depend upon meteoric or telluric miasmata and affect only single individuals who happen to be predisposed for such diseases at the time when they are prevalent;

(b.) Endemic (morbi endemici). These diseases are confined to a definite and often very limited region; they are distinguished from the former by being dependent upon the situation of a place and its surrounding region, upon the climate, the condition of the atmosphere, winds, soil and water, upon the mode of life of the inhabitants, food, social life. Every place may therefore have diseases which are peculiar to it; it is a remarkable fact that apparently identical diseases which prevail in places not very distant from one another, require the application of different remedies in the different places.

(c.) Epidemic (morbi epidemici). These diseases are closely related to the former, with this difference, that they prevail at periods in a greater or lesser extent of country, and affect all ages and sexes indiscriminately; they depend upon a cause of atmospheric or cosmic origin, generally upon a miasm which becomes contagious among crowded masses and then spreads so much more rapidly and over a larger surface.

(d.) Morbi annui. These are diseases which prevail at particular periods of the year, in the spring, summer, fallor winter. In the winter inflammatory diseases are prevalent, whereas the prevalent diseases in spring are rather of a catarrhal nature. The fall diseases are characterized by gastric-pituitous symptoms, and those of the summer-season have moreover a typhoid character. This class of diseases is evidently affected by sudden changes in the weather.

(e.) Morbi stationarii. It is of great importance to a physician to know what peculiar character a disease is disposed to assume in a place. This topical influence modifies the character of the above-mentioned diseases more or less. It prevails during a shorter or longer number of years, abates gradually, finally disappears entirely and returns after an indefinite period.