various origin. We have already mentioned the innumerable varieties living in the ground, in the water, and on plants, which play so many important parts. Some of them may, under many circumstances, borrow a pathogenic power and produce diseases. There are also others, normally pathogenic, which have been eliminated from diseased organisms, and instead of succumbing at once they have fallen into the outer world, have adapted themselves to the new medium, and are living another life in the ground or in water. They are all ready when, with food or by respiration, or by a scratch of the tissues, they enter a living organism anew, to determine in it, if circumstances are favorable, the disease characteristic of them. So do the microbes of cholera, tetanus, etc. Social influences play an important part also from this point of view. All kinds of microbes may be carried to long distances by the solid matters of every kind that are employed in innumerable ways in the life of society. The solids may transport the microbes just mentioned as living in the external medium, and also those which come direct from a diseased subject. This distribution of agents of infection by solids is of extreme importance, but has attracted attention only within a few years. The hands may retain infectious germs and carry them to a long distance, often without the person carrying them being affected. Examples are abundant that illustrate the transportation and propagation in this way of pyogenic and septic infections, erysipelas, etc. Clothing, carriage cushions, tapestries, and bedding may preserve and carry cholera, smallpox, scarlatina, diphtheria, and erysipelas. The most various utensils, food, and particularly bread, may be soiled by pathogenic microbes, and thus facilitate their penetration into the organism.
We may understand, therefore, without having to insist upon it, how a large number of social circumstances may expose persons who live in society to the attacks of microbes. One's occupation will often force a person to come into contact with patients afflicted with infectious disorders, or with excreta from such patients containing pathogenic microbes, and thus cause him to contract such diseases as cholera or typhoid fever. Occupations having to do with diseased animals may also expose those who are engaged in them to direct infections, as when a groom takes care of a glandered horse; or to indirect infections, as with tanners preparing the hides of animals that had anthrax.
These examples show that there are extremely multiplied processes that may expose men living in society directly to infection by microbes, while mechanisms not less complex and equally of social origin may prepare the organic ground for the invasion of the microbe by changing either the structure or the working of the organism.
