CHAPTER XVII.

VITAL CAUSES OF DISEASE.

THE vital causes of disease are those vegetable and animal organisms which are capable of existing as parasites in or upon the human host, and by their mere presence, or through the production of poisonous products, cause disease. The vegetable organisms producing disease are of two classes-the bacteria, and various fungi which produce local affections of the skin. Fortunately, only a small proportion of the bacteria found in nature are pathogenic. A number of the specific diseases, however, have been traced to the activity of these organisms. All of these pathogenic organisms are transmissible from one individual to another if not already immune. One attack of certain of these specific diseases confers immunity against subsequent infection from the same species of organism. In some instances this immunity is permanent, lasting throughout life, while in other diseases it is of short duration, lasting only a few weeks or months. The specific organisms of each disease differ in their morphologic and biologic characters from those causing other diseases to such an extent that they can be recognized and isolated.

The bacteria which are pathogenic for man, and the specific organisms of the following diseases, belong to the vegetable kingdom, and have been discovered and isolated as follows:

1839, parasite of favus;

1863, bacillus of anthrax-Davaine;

1873, spirillum of relapsing fever-Obermeier; 1875, bacillus of malignant edema-Pasteur; 1878, actinomycosis-Bollinger;

1879, micrococcus of gonorrhea-Neisser;

1880, micrococcus lanceolatus (pneumonia)—Sternberg; 1880, bacillus of typhoid fever-Eberth;

1881, micrococcus tetragenus-Koch and Gaffky; 1882, bacillus pyocyaneus-Gessard;

1882, bacillus of tuberculosis-Koch;

1882, bacillus of glanders-Löffler and Schütz; 1882, bacillus of rhinoscleroma-von Fisch;

1883, streptococcus pyogenes-Fehleisen;

1883, bacillus lanceolatus-Friedländer and Frobenius; 1884, vibrio of Asiatic cholera-Koch;

1884, staphylococcus aureus and albus-Rosenbach; 1884, bacillus of diphtheria-Klebs and Löffler; 1884, bacillus of tetanus-Nicolaier;

1885, staphylococcus citreus-Passet;

1885, bacillus proteus vulgaris-Hauser; 1886, bacillus coli communis-Escherich;

1886, bacillus lactis aërogenes-Escherich;

1887, micrococcus intracellularis meningitidis-Weichselbaum;

1889, bacillus capsulatus-Pfeiffer;

1892, bacillus of influenza-Pfeiffer;

1894, bacillus of bubonic plague-Yersin and Kitasato; 1894, bacillus of infectious conjunctivitis — KochWeeks;

1895, bacillus botulinus-Van Ermengen;

1897, bacillus enteritidis sporogenes-Klein; micrococcus melitensis;

1900, bacillus mortiferus-Harris.

In the following diseases no specific organisms have ast yet been isolated, though from their clinical manifestations and contagious character they are believed to be due to some specific agent: Small-pox, varicella, measles, scarlet fever, whooping-cough, mumps, dengue, typhus fever, yellow fever, rabies, and rheumatism. A variety of micro-organisms has been found in most of these diseases, though none of them has been positively de-. monstrated as specific.

Modes of Dissemination.-Those diseases which are infectious may be disseminated in several different ways. The confusion which is more or less prevalent with regard to the exact term to employ in each disease, whether infectious or contagious, has led numerous writers to abandon one or the other term. Since all contagious diseases are infectious, it seems preferable to abandon both, and to substitute the term transmissible, as has been done by Dr. Abbott,' and call those diseases which were formerly designated as contagious, transmissible by direct contact, and those diseases which were formerly designated as infectious, transmissible by indiDiseases like small-pox, measles, and scarlet fever are transmissible by direct contact; and diseases like typhoid fever are usually transmissible by indirect.

rect contact.

contact.

Some of the diseases, however, which are frequently disseminated by direct contact may, and often are, disseminated indirectly through the medium of food infected with the specific micro-organisms, or through the agency of flies or other insects whose bodies have become infected by coming in contact with infective materials. A number of diseases, prominent among which are those due to animal parasites, are disseminated through the use of meat derived from infectious animals, through water polluted with animal excrement, or through green vegetables that have come in contact with such excrement.

In addition to diseases transmitted by these modes of dissemination, there are some diseases in which the infective agent passes one of its cycles in the body of an insect, and is disseminated through the bite of such infected insect. The disease which is best known as being disseminated in this manner is malaria, which is disseminated through the bite of infected anopheles, while filariasis is believed to be disseminated through the bite of another mosquito, a species of culex, and the latest studies upon yellow fever indicate most strongly The Hygiene of Transmissible Diseases.

that this disease is disseminated through the bite of another variety of the genus culex.

In the light of the discoveries in regard to the mode of dissemination of malaria, filariasis, and yellow fever by means of mosquitoes, and the dissemination of Texas cattle fever by means of the cattle tick, it is safe to predict that other diseases will be found to be disseminated by somewhat similar agencies. There is no doubt that in the immediate future the investigations of scientists will be directed toward the discovery of such disseminating agencies, and with the discovery of such agencies the causes of these diseases, as yet undetermined, may likewise be discovered, and our measures of prevention placed upon a more satisfactory basis.

Nature of Epidemics.-The opinions of authorities differ with regard to the number of cases of any infectious. disease that may exist in a community before the disease may be declared to be epidemic in its character. A few isolated cases are usually spoken of as an "outbreak " of the disease; but when the number of cases amounts to I per 1000 of the population, it is usually said to be epidemic. In England, measles is said to be epidemic. when the number of cases amounts to 1.2 per 1000 of the population. In olden times yellow fever was said to be epidemic when the number of deaths from that disease. exceeded the number from all other causes. At the present time the disease is considered epidemic in New Orleans when there are between 2000 and 5000 cases, or about 22 per 1000 of population. Usually when 10 cases. of any disease develop in close proximity to each other, when all sanitary precautions have been exercised, the disease is said to be epidemic.

A disease is said to be epidemic when the infection has been imported into a locality and spreads over an area. A disease is said to be endemic when it develops within a locality, or is peculiar to a locality, and spreads over an area. A disease is said to be pandemic when it spreads over very large areas or prevails in several continents at

the same time. The term pandemic disease is usually applied to cholera, yellow fever, influenza, and plague. The different pandemic diseases have local habitats from which they are rarely or never absent. The habitat of influenza is in Russia, that of cholera in the valley of the Ganges River, that of plague in Indo-China, and that of yellow fever in Cuba.

Immunity and Susceptibility.-When we undertake to investigate the causative factors underlying the results of the exposure of an entire community to a source of infection, with the idea of ascertaining the reasons why a certain proportion of such a community escapes the infection, a number of questions of great practical importance present themselves. We are frequently satisfied in solving these questions by saying that those members of the community which escaped the disease did so from the fact that they were not susceptible, or, probably, that they were immune, and that those who contracted the disease were susceptible, or that they were not im

mune.

It will be necessary to consider briefly what is meant by the terms susceptibility and immunity, and then we shall better understand why such differences in effects. manifest themselves.

Immunity may be described as that condition of the body in which the organism resists the invasion of disease-producing bacteria, or resists their growth and activity after they have gained an entrance; while susceptibility is the opposite condition, in which, instead of resistance, there is a passive inertia, which allows the disease-producing bacteria to develop.

Immunity is either natural or acquired. Natural immunity is the inherent, vital, reactive state of a healthy organism against the invasion of foreign agents. This natural resistance to the invasion of disease-producing bacteria is to some extent a racial condition, as seen in the greater susceptibility of some races to certain diseases than others. It is also to some extent a family