foundation of advance in the prevention of disease. There still remain more worlds to conquer. Much has yet to be learned of the causal agent of typhus, of scarlet fever, of measles and sleeping sickness, and chief of all there remains the great problem of cancer.

But the discovery of the germ by the bacteriologist was only the first step in the process of prevention. Let me take one or two examples. The typhoid bacillus was discovered by Eberth in 1886. Quite rightly the avenue of infection was assumed to be the alimentary tract. It was realized at once that water was the chief means of infection and that water became the vehicle through contamination with excrement from cases of the disease. Immediately the necessity for a pure water supply, that is a supply free from the chance of infection with sewage, was obvious. In this Britain took the lead, as indeed was most natural seeing that physical geography made the securing of pure water supplies comparatively easy for her. America and the continent of Europe lagged behind. The cities of Britain became typhoid free, while Montreal, New York and Paris continued to suffer. Gradually methods of filtration and chlorination were introduced. Advances in civil engineering rendered possible the formation of large reservoirs collecting upland waters at long distances from great cities. But still unexplained epidemics arose. Then in the early days of this century it was realized that the bacillus of typhoid did not remain in the intestinal tract. At an early stage it entered the blood where it could be demonstrated and from which it wandered into odd corners. It was found that the gall bladder was one of these. In contrast to most germs the typhoid bacillus is not destroyed by bile. It actually flourishes in it. In this situation it may remain latent for twenty years or more, setting up at most a slight catarrh of the passages, often causing gall stones but not otherwise inconveniencing the harbouring individual. But while relatively harmless to the individual who carried it the germ continued to be voided in the excrement and so the carrier continued to be a focus of infection. Some 20 per cent. of those who had suffered from typhoid fever became carriers of the disease for a time in this way, but only some

three or four per cent. were chronic carriers. Thus it was not sufficient to segregate the sick. It became necessary to eliminate all possibility of human contamination of water supplies or else to chlorinate or otherwise disinfect the water and so kill the bacteria. There are of course other media of transference of the disease. In the South African War, carried on in a climate dry and warm, dust and flies contaminating food as well as water assumed importance. Well was it for civilization that preventive medicine had advanced so far before the great war. In the South African campaign there were twenty cases of typhoid to every one of wounds. Had the same proportion obtained in the recent war the strain upon our hospitals would have been beyond coping with. Thus preventive medicine may be regarded as the saviour of civilization—the factor which won the war.

Take another example, the case of plague. Now plague is a very ancient disease. In all probability the affection from which the Philistines suffered when the ark of God was amongst them was plague, and it looks very much as if the modern discovery of the implication of the rodent in the transmission of the disease was known in Bible times. When town after town of the Philistines had been attacked by the plague they eventually decided to send the ark back and in it as a trespass offering five golden emerods (the buboes) and five golden mice, the rodent source of the disease.

Plague is not now the menace it was in earlier times. In the reign of Justinian fifty per cent. of the population of the Roman Empire were wiped out by plague. Plague was the black death of the middle ages when in one year (1348) it destroyed one-quarter of the inhabitants of Europe. In 1665, according to Pepys's diary, plague accounted in London and neighbourhood for 70,000 deaths. Since then it has at stated periods swept across large areas of the earth's surface, notably in recent times India, where in ten years it accounted for six million deaths, and China. But the mystery of these visitations was not cleared up until within the last twenty-five years.

The plague bacillus was discovered by Kitasato and Yersin in 1894, the year of the Hong Kong outbreak. It was realized that the disease could be transmitted through the

skin, the lung or the bowel. In by far the greater number of cases infection occurred by the skin, the disease showing itself first in the glands, usually glands of the groin, the enlargement being called a bubo, hence bubonic plague. For a long time it was thought that cracks or abrasions in the skin were the points of entrance. Then it was noticed that an epidemic amongst the rats preceded the outbreaks in the villages of India. Suspicion centred on the rodent and it was suggested that the rat flea might be the carrier. Monkeys could be readily infected with plague. The interesting experiment was then made of putting monkeys in cages, surrounding these cages with fly paper of varying breadth and putting a rat dead of plague just beyond the fly paper. If the rat was more than five inches away from the cage no infection of the monkey took place and the fleas were all found on the paper, the maximum long jump of a rat flea being five inches. If the rats were placed within the five inch radius then the jump was within the compass of the flea and the monkey became infected. Now it has been ascertained that the disease is essentially one of rats and other rodents, including the native ground squirrel of California, and that whenever these animals are affected there is the possibility of transmission to the human subject; but this transference seldom occurs except in what are called 'mediaeval conditions', i.e. when there is much refuse, many rats and filthy human habitations. Preventive methods therefore include a study of rodent life, extermination of rats with poison, rendering houses rat-proof, removal of garbage, fumigation to destroy the flea, and so on.

Tuberculosis may be taken as an example of a different kind of disease altogether. The germ was discovered by Robert Koch in 1882. It had already been recognized to be a disease affecting animals, more especially bovines, as well as man. It was assumed that infection mainly occurred by the air passages but also by the alimentary tract from infected milk and meat. This latter point was called in question by Koch in 1901, but the work which was stimulated by his pronouncement has confirmed the view that a large amount of tuberculous disease in children, at least 25 per cent., is due to infection with the bovine bacillus introduced by con

taminated milk. Curiously enough the amount of this infection varies in different parts of the world. It is apparently greater in Britain than in America and particularly frequent in Scotland. The bovine bacillus affects only certain parts of the body, the abdomen and bowel most commonly, but very frequently also the bones, joints and glands of the neck. It almost never settles primarily in the lungs. Thus consumption in the usual sense of the word is almost never due to milk infection. There are those who would look on calmly and take no special precautions, regarding the risk which the child has to run in early infancy as a means of immunizing it against subsequent human infection, but that is a most dangerous attitude to adopt. Control of the milk supply for this reason above all others is part of the duty of the health department of our cities and towns. But unquestionably the large amount. of tuberculous infection, even some of the alimentary infection, is human in origin. The bacillus presumably enters at some point in the respiratory tract, but where this actually is, whether high up in pharynx or tonsils or low down within the lungs themselves, is still a matter of acute controversy. One point however stands out, a point emphasized only comparatively recently, that some eighty or ninety per cent. (some would make the percentage higher) become infected at some time during childhood or adolescence. Of course in the vast majority of instances the disease goes no further than a small area in the apex of the lung which subsequently heals. What is the source of the infection which is so widespread? Undoubtedly the tuberculous individual, the consumptive, who has an open lesion in his lung and who is expectorating infective material and contaminating cups, spoons and other household utensils. Preventive measures are obvious-proper housing conditions. What can a family which has to live in a house of two or three rooms do to prevent the disease spreading amongst them if one of their number is tuberculous? Proper ventilation. Why is tuberculosis so terribly common in the magnificent air of the Outer Hebrides? Why, simply because in most cases the windows of the houses are not made to open, and, man, woman, and child, the people spend more than half their life indoors. Segregation of the infected at the

earliest possible moment, and the prevention of spitting in public places, the elimination of the community drinking cups, etc., and other obvious means of prevention.

It would take me too long to deal with the great social scourges-the venereal diseases. They require really a paper to themselves. But there are one or two of the well-known diseases of the warmer climates which are of special interest in their preventive aspects. Malta fever is one of these. This is a disease which although not very fatal lasts a very long time, sometimes for months. It was peculiarly annoying to the Army and Navy in Malta and the Mediterranean on this account. The small round germ which causes it was discovered by General Sir David Bruce of the Royal Army Medical Corps in 1887. But until 1905 the most exhaustive investigations failed to reveal the source from which the germ was obtained. The air, the dust, the water of harbours and of wells, mosquitoes-all were searched for evidence of the germ. Then the whole puzzle was cleared up through the voyage of an American sailing ship, the Joshua Nicholson, from Malta to the States with a number of Maltese goats on board. The ship's crew became infected in large numbers and the only possible source the interval being too long for shore infection-were the goats. Attention was drawn to these and it was found that 50 per cent. of the goats in Malta showed evidence of the disease germ in their blood. Preventive efforts were concentrated on the goat and its milk, and the result was that the cases in Malta fell from 643 in 1905 to seven in 1907. The custom in Malta at one time was, if you wished some café au lait to seize a passing goat and help yourself. Now this custom is discouraged by the authorities.

The story of how preventive measures against yellow fever were discovered is an epic. Known already to many, it will bear repeating how, as Sir William Osler put it, from being a death trap Central America has become a health resort and the Panama Canal an accomplished fact. Students of history will recall the countless occasions on which crews of ships, expeditions and armies were wiped out by this unseen mysterious agent. Until quite recently medical men spoke of a miasma emanating from swamps and low lying marshy