The poisons, most notably those of cholera and enteric fever, which are got rid of from the body by means of the intestinal canal, get into sewage, and so the sewage gets infected. The poisons are then spread to human beings in one of two ways, either by sewage getting into the water which human beings drink, or by the poisons, through the shaking about of the sewage and the decomposition that goes on, being carried up bodily into sewer air. Some very instructive experiments made by Dr. Frankland have shown that particles may be carried up and suspended in the air. These particles may be carried up and suspended in the air in the sewers and drains, and by methods of communication get into the houses, or into the drinking water by means of the waste-pipes of water cisterns.

These poisons may be carried about by rats, which make their way from bad drains into the basements of houses, and to different parts. This is a method of communication of these diseases which has been too much overlooked. In my opinion the presence of rats ought always to be considered dangerous, as not only may the rats carry the poison of these diseases, but wherever a rat can go foul air can find its way, and that air may contain the poison of one or other of these diseases. This, I believe, is really a very common way in which these diseases are spread.

Another method, which I have long been of opinion is a very important one, and one that ought not to be overlooked, is, that these poisons may be carried about by flies. It has been shown by a very admirable series of experiments made in France by Dr. Eaimbert, that the poison of malignant pustule, a disease which affects cattle, can be carried about by flies, is commonly carried about by them, and by that means may be communicated to human beings. Eaimbert, moreover, examined the poison, allowed flies to settle upon poisoned materials, caught the flies, examined them, and found substances containing these poisons upon their feet, and he allowed these flies to infect animals, and showed that these poisons passed readily through the mucous membrane, and were readily absorbed through the mucous surfaces, and in one case through the skin, thus proving that it was not necessary, as was commonly supposed, for an animal to be wounded in order to take the poison. He further showed that the flies that did this were not, as commonly supposed, stinging flies, but, on the contrary, common house flies, blow flies, flies provided with probosces, commonly regarded as not only harmless, but as scavengers to remove filth.

This matter has not been taken up at all, but I noticed, I think, three years ago in a number of the Lancet an account of an epidemic of smallpox in a place which was observed at the time to be accompanied by the inroad of an enormous army of flies. It struck me at that time that it was just possible that in that instance, and in one or two other instances, it may have been spread by flies. It has struck me since much more forcibly in noticing the way in which flies abound in hot weather in hospital wards, and settle upon suppurating wounds, and fly from one patient to another, settling upon different parts of the body, especially upon the orifices where the mucous membrane comes to the surface, that it is quite possible that flies may disseminate these communicable diseases, especially such as erysipelas, hospital gangrene, and perhaps smallpox, in which there are crusts and scabs, which the flies may carry with them in a way which has been entirely unsuspected.

Now, the poisons of these diseases are separated from the bodies of infected persons in different ways; in almost all cases the poison is eliminated from all the excretory organs, but in certain of these diseases it is eliminated by one or other of the excretory organs by preference. For instance, in the case of smallpox, scarlet fever, and measles, it is got rid of from the skin and certain mucous membranes. In scarlet fever and diphtheria it is got rid of from the throat; in typhus fever it is got rid of by means of exhalations of the breath, and from the skin. In whooping cough it is coughed into the air by exhalations from the lungs ; by the mucous membranes of the lung passages it is breathed out into the air. In enteric or typhoid fever, as in cholera, it is almost entirely got rid of by means of discharges from the intestinal canal. Some of these poisons, you see therefore, leave the person infected in such a manner as to get into the air around, and into the bedclothes, in a kind of fine cloud of dust, although it is true that in all these diseases the contagious atmosphere is only close round the patient, and the disease is not communicated from one person to another except at a small distance. In the case of the other diseases, such as cholera and typhoid or enteric fever, in which the poison does.not get out into the air around, but leaves the body by means of discharges from the intestinal canal, these diseases have often not been recognised as contagious or communicable.

What I want you to understand is that the difference between the communication of the poisons of these two classes of diseases is not in the poison, or in the nature of the poison, but in the way in which the poisons leave the body of the infected person. I want you to come to the conclusion that the last mentioned diseases, cholera and typhoid fever, are just as essentially communicable diseases as scarlet fever and smallpox.

The diseases which are most obviously communicable have among them diseases which it is supposed every one must take. They are diseases that especially belong to children ; the reason is that they are diseases which a person only has once in his life as a rule. So you see they especially affect children, because as soon as a child is exposed to the poison of one of the diseases he is most likely to take it, in which case he does not, as a rule, get it again later in life, and if he does not take it then, it is very likely that he will never have it at alL The only one of these diseases that is no longer specially confined to children is the most dreadful of them-viz. smallpox, although it was formerly the most fatal disease of childhood, and killed half the children under ten years of age before vaccination was practised It is now rare for a child who has been properly vaccinated to have smallpox.