It has long been observed that the phenomena of these diseases resemble in various ways the phenomena which occur during decomposition or putrefaction of organic substances; and it has been shown by several observers, notably, you will remember, lately by Professor Tyndall, that putrefaction does not occur unless certain material particles are in the liquid, or get into it from the air.

One resemblance of these phenomena to putrefaction that I will point out is, that the poisons of these diseases and the bodies which produce putrefaction alike multiply indefinitely in suitable media.

Another thing to be mentioned about these poisons, to put it in homely language, is, that they always breed true, that the poison that comes from small-pox is not capable of producing scarlet fever, or that the poison from a child suffering from measles does not give another child whooping cough or diphtheria; and so you see that there is something definite about each of these poisons, which gives it a character that we call specific; hence these diseases go by the name of specific diseases. Lastly, these poisons are destroyed by suitable agents; for instance, we have a large amount of evidence supported by remarkable scientific experiments, that heat, a little above the boiling point of water, will destroy the poisons of these diseases, and that is another analogy between the poisons of these diseases and the agents which are present during putrefaction, which we know are living things; so we can gradually, without ever actually seeing them, arrive at the conclusion that the poisons of these diseases moat resemble the bodies that are present during putrefaction, and that like these they are living things. If we accept that we come to the conclusion that these poisons are, in the first place, actual particles of solid matter; in the second place, that they are organic; and in the third place, that in all probability they are living things; and in connection with their being living things, I must point out the important consideration that the first stage in all these diseases is the stage of incubation; that, after a dose of the poison, whatever it is, has been, taken, the disease does not develop itself until that poison has had time to grow. Now, contrast that with the action of any ordinary poison that you know of; for instance, if I take a dose of prussic acid, it does not wait for three or four days before it kills me; it kills me instantly, or almost instantly; and the same can be said of arsenie, or organic vegetable poisons, as strychnine, or morphia; a sufficient dose of either kills a person almost instantly; it takes merely the time during which it is being absorbed; so you see that the poisons of these diseases differ in a very peculiar and remarkable manner from other poisons with which we are acquainted.

We will consider first what we know about the origination of these poisons; what we know about the media by which they are conveyed from one place to another; what we know about the circumstances which favour the spread of one more than another, and the methods for the prevention of their spreading.

It has been maintained that every one of these poisons may originate at any time and anywhere under suitable circumstances. One of the greatest of physicians, Dr. Trousseau, said that the fact that all these diseases must have originated some time or another somewhere shows that they may originate at any time anywhere if conditions are suitable; that appears a logical way of putting it, but still that statement implied a theory which requires a great deal of proof. You may almost as well say that wheat originated at some place, and that, therefore, it may originate elsewhere under suitable conditions, the only difference being that grains of wheat are larger things than the poisons of these diseases. But you will now find very few people who will go so far as to tell you that the poison of any one of these diseases may originate at any time anywhere; as a matter of fact, the majority of people believe now that the poisons of most of these diseases do not originate at any time anywhere, but come from a previous case; that scarlet fever, for instance, is not got by anybody, unless he somewhere or another,, has come into communication with the poison derived from a case of scarlet fever.

I want you to notice, that one after another of these diseases has been put out of the category of diseases which arise spontaneously, and it has now been brought down by almost all observers to the diseases known as typhus and typhoid (or enteric) fevers, with a few others of less importance. It is admitted that these diseases are communicable from one person to another, with the single exception of typhoid or enteric fever; but there are persons who believe that such communication does not occur in the case of typhoid fever, or if it does, it is a very rare thing, so we may say that it is admitted by most persons that the majority of these diseases are only caught by contact.

The poisons of these diseases are carried about suspended in the air, in the dust, and, so far as we know, may be carried considerable distances in this way, hut in most of the diseases the atmosphere around a person who is suffering from the disease does not give it to the persons in the same place beyond a very small distance. It was observed some time ago, in the last century, that the atmosphere around a smallpox patient is only infectious at a little distance, and in typhus fever the distance is still smaller; that, in fact, the poison given off into the air around became so diluted in the air that it is at any rate not usual for persons to take the disease except at a very short distance from a person suffering from one of these diseases.