the culture was weakened and a mild form of the disease would be produced. History shows few incidents more dramatic than the demonstration by Pasteur that anthrax could be given a chicken if its bodily temperature were reduced and that it would speedily recover when the temperature became normal. This principle he developed until in 1885 he was able to give the first inoculation against hydrophobia (rabies). At a much earlier date he had satisfied himself that disease was largely due to bacteria and that the great death rate in hospitals from operations could be lessened by greater care in the handling of bandages and in the sterilizing of hands and instruments. Not a physician himself, he effected, nevertheless, an epoch-making revolution in the practice of medicine and surgery.*

Koch

Meantime there was working in Germany a man who ranks with Pasteur. Robert Koch (1843-1910) attracted attention in 1876 by his studies of anthrax and by his development of a method of obtaining a pure culture of pathogenic organisms. He proved that the Bacillus anthracis was the sole cause of the disease. A few years later he was satisfied that the infections following wounds were due to microorganisms and urged the necessity of identifying and isolating them. In 1882 he made the announcement of the discovery of the bacillus causing tuberculosis. In 1883 he found the cause of Asiatic cholera and expressed his belief that insects were often carriers of disease.

The Germ Theory

At first medical men were often disposed to be skeptical about the discoveries of these last named men. Even as late as 1896, when the writer was a student at Berlin, he heard a lecture given by a so-called scientist in which Koch was condemned as having made German science ridiculous in the

eyes of the world. Yet Koch imposed very severe tests upon himself and the accuracy of his studies soon carried conviction. He said that before we could be certain that a given germ was responsible for a disease it was necessary: (1) to find it present in large numbers in every case; (2) to make a pure culture of it in some medium; (3) to inoculate a subject with the pure culture and produce the disease; and (4) to find again large numbers of the germs in the inoculated subject.

To-day the scientific world does not question the belief that all diseases which can be transmitted from one organism to another are caused by some other organism usually microscopic in size. After the start was made there followed a series of discoveries of specific organisms which have been shown to be the causes of the diseases of man and of his domestic plants and animals. The list is far from complete and does not include some of the commonest diseases like measles. It may be that some of the undiscovered germs are too small to be seen with the best microscopes. Nevertheless the discovery of the nature of communicable disease has completely changed the problem. Now when we find the culprit we ask such questions as: Where does it live? Under what conditions does it increase? How does it get into the body of host?

The Pathogenic Bacteria

The original home of the bacteria seems to have been the upper inches of the soil, where they are still found in the largest numbers. As animals walked over the ground numbers of bacteria were picked up and carried on the surface of the body. Some of them found there a residence suited to their needs and remained. The host is rarely conscious of their presence nor can it be said that he is injured by them. If, however, there is a cut or accident making a break in the skin, the same bacteria may enter and cause some damage. Acne on the human skin is caused by a bacterium ordinarily

living on the surface but if in any way it gets deeper into the tissues boils or carbuncles are produced which may be serious. Through food, breath, and drink other bacteria have reached the mucous membranes of the body and there reside, again doing little harm under ordinary conditions, but if there be injury or if the tone of the body be lowered, a condition favorable to their growth is produced and a cold, or pneumonia, perhaps, develops. Some of the bacteria are pusproducing if they get into closed wounds, hence the danger of all deep cuts. These bacteria which have come to live on or in any given animal will be transported wherever the animal goes. In any case there is little likelihood of getting rid of the uninvited guest. Other bacteria have a more complicated life cycle and must live for a time on the earth, or in some insect or molluscan host ere they can be transmitted to the mammal and produce disease. This is true of many parasites besides bacteria. Thus the broad tapeworm, which is rather common in folks about the Baltic, produces eggs which escape from the body of the host into running water and hatch. The young are then eaten by minute crustaceans called Cyclops (the water flea) which in turn are devoured by perch or pike. If man catches the fish and eats it without thorough cooking there is a chance for the worm to develop. In the Orient there are several flukes, related to the tapeworms, which pass from snails to crabs or fish and thence to man. Ordinarily, then, the parasitic organisms must enter the body through the nose or throat and must leave the body through the excretions. The skin is a protective covering which stops most of the enemies unless broken in some way as by accident or the bite of an insect. The germs of tetanus are common in heavily manured ground and find their way through some little break in the skin. On the other hand, the germ of the hookworm disease is able to burrow through the skin itself. It is evident that we are again confronted with the web of life and that it is to man's advantage to upset

the balance of nature if he can do so. In this case the interrelations are so complex that he must know them ere his moves are likely to be effective.

It has been noted in an earlier chapter that there are certain limits to the existence of any form of life and a certain optimum under which any given form prospers. There is, thus, a geographical distribution of disease. Those bacteria which live in so far as we now know only in man or his animals, such as smallpox and tuberculosis, may be and probably will be carried wherever he goes. Yellow fever is limited to the tropics save during summer time and it was early noted that it disappeared from the United States with the coming of frost. Malaria is a serious curse from Baltimore south but is negligible north of New York City. Bacterial diseases, then, are likely to be more numerous in the warmer and moister regions of earth and to be least in evidence in high mountain countries and polar regions. This corresponds to observed facts although we should have to include density of population to make the picture wholly accurate.

THE CONQUEST OF SPECIFIC DISEASES

In as much as a general survey of the problem of disease is out of the question, let us consider a few of the chief diseases of the temperate regions.

Smallpox

Fortunately for man he has been able in some cases to gain a mastery of disease without exact knowledge of its cause. The most striking illustration is smallpox. Few men of the present generation have any idea of the havoc wrought by this disease among our European ancestors nor of the danger that it presents to us to-day. Throughout the Middle Ages it killed not less than one-tenth of the entire population and few there were who escaped it during their lives, from 80 to 90 per cent having it at some time. Few even among

the royal families had faces free from pockmarks. In 1722, the town of Ware, England, contained 2,515 inhabitants of whom 1,601 had been victims of the disease. An epidemic came and only 302 were left untouched. In Iceland, in 1707, some 18,000 of a total population of 30,000, or 60 per cent, died in a single epidemic. In 1752, Boston, England, had a population of 15,684 of whom 5,998 had survived an earlier attack. An epidemic came and some 5,545 contracted the disease, while 2,124 were directly inoculated with it following a custom introduced by Lady Montague in 1721. Deducting those who fled, only 174 remained who had not been sick. One-third of the passengers on the boat bringing William Penn to America are said to have died of smallpox en route. In the Revolutionary War from one-fifth to one-fourth of the army was sick at times, chiefly from smallpox. In the Union forces during the Civil War there were 12,236 cases and 4,717 deaths, a death rate of 38.5 per cent.

"A pox upon you" was one of the great curses of England in days gone by. Edward Jenner, a London doctor, having noted that the milkmaids usually had smooth faces, wondered if there could be any connection between the little sores they often had on their fingers, known as cowpox, and their freedom from smallpox. He inoculated a boy with cowpox in the year 1796 and then exposed him to smallpox. This was the beginning of vaccination and of the first great victory of man over his bacterial enemies. At this time 18 of every hundred deaths in London were due to smallpox. Ten years later this rate was reduced to 6 and in another decade to 3. Some men are mild, others are fierce. Bacteria are the same. The vaccine virus is nothing but a benignant type of the bacteria produced by breeding in a domestic animal. When transferred to man a small sore results and immunity to the later attack of the violent type is given. The results speak for themselves. In 1,166 cases of the Union men in the Civil War it was noted whether they had been vaccinated or