Poisons, in the ordinary sense of the word, can scarcely be said to produce diseases. Some of them, indeed, are chemical agents, such as alkalies and acids, which act locally on the tissues, and produce inflammations in the usual way. Others, such as arsenic and cantha-rides, produce local inflammations, not by any obvious or gross chemical action, but by some more intimate influence on the vitality of the structures. These two agents, like many other poisons, whatever the portal by which they find entrance to the body, being carried by the blood, select certain localities on which to act. Arsenic acts as an irritant on the mucous membrane of the stomach and intestines, producing the phenomena of inflammation there; cantharides acts similarly on the kidneys and bladder. Most other poisons resemble these two in-respect that they select a certain locality for their action, and that they produce their effects by some intimate influence on the vital processes of the living tissues, this influence probably having to do with the finer chemical processes concerned in the vital phenomena. In the case of most poisons, the nervous system is more or less affected, many of them having special kinds of actions, and some of them selecting special parts of the nervous system. Thus, strychnia acts as an irritant upon the motor centres of the cord and medulla oblongata. The term Intoxication (which literally means poisoning) is used to designate the effects produced on the body generally and more particularly on the nervous system by poisons.

Whilst the phenomena produced by poisons introduced from without can scarcely be regarded as diseases, the same does not apply to poisons produced in the body itself. Most of the disease-producing microbes bring about their effects by means of poisons, and Intoxication is an almost constant effect of diseases so produced. In some cases there is absorption of poisons from the contents of the alimentary canal, in which case the term Auto-intoxication is used. Again, the metabolism of the tissues may be associated with the evolution of poisonous products, in which case also the term Auto-intoxication is applicable. The toxines so produced, like ordinary poisons, have, in their action, affinities for special localities of the body. . 3. Infective agents stand in a totally different position from ordinary poisons. The term is used in relation to agents, which, when introduced into the body, increase by self-multiplication, and produce effects altogether disproportionate to their original amount. They may be introduced in exceedingly minute quantities, but they may have very severe effects, resulting even in death.

These agents owe their special characters to the fact that they consist of the living cells of minute microscopic organisms. Most of them belong to a great division of the vegetable kingdom, and are grouped under the general terms, Bacteria, Micro-organisms, and Microbes. In a few cases, and especially in malarial fevers, minute animal organisms have been discovered. These minute organisms, whether vegetable or animal, propagate chiefly by simple division of their cells, and do so generally with great rapidity, so that they multiply very greatly in a limited time. They do not usually produce their effects directly, but by means of poisons eliminated by them in the course of their vital processes.

Some of the microbes produce in the first instance purely local effects, these effects having as a rule the characters of inflammations, sometimes with special characters which serve to distinguish the various kinds of agents. Thus syphilis and tuberculosis always begin by the local application of the morbid poison, and their primary phenomena are inflammations more or less modified. But even those which have a primary local seat very readily extend their influence beyond it to the circulating blood, while many forms appear to have no primary local seat, but immediately pass into the blood. It is so with typhus fever, yellow fever, scarlet fever, and others. This extension to the blood may be of the infective agents themselves, the bacteria propagating into the blood and multiplying there, or it may be a mere filtration of the poisonous products of the microbes, the latter remaining local. Thus tuberculosis may remain entirely local, and yet fever is a nearly constant result from the toxine passing into the blood. Indeed there are some forms which apparently never pass beyond their local seats and yet produce general phenomena. Thus the microbe of cholera appears to confine itself to the intestinal canal, but its products are absorbed and produce effects upon the nervous system and otherwise.

The term Infective Disease is applied to affections of this class with reference to the fact that the disease is communicated by infection, that is to say by the application of such agents as those referred to above. The term Contagium or Contagium vivum is frequently used in a sense equivalent to infective agent. The mode of extension of this class of diseases amongst the community is somewhat various, and in some cases difficult to determine. There are many in which the disease is simply passed from person to person, either directly or by clothing, excreta, or otherwise. Such diseases are called distinctively Infectious or Contagious.

But the mode of extension is in many instances not so simple as this. There are some infective agents which appear to reside in special localities, being apparently fostered by the special conditions of temperature, moisture, and other factors in these localities. In order to acquire such affections a person must visit the locality, and the morbid poison must, as it were, rise from the ground or proceed from its local habitation and pass into his body. The term Miasma is applied to such an infective agent, and the diseases are called miasmatic. The most typical instance of it is malarial fever, although there is reason to believe that acute rheumatism, and some other affections are miasmatic.

To the class of disease here under consideration the terms epidemic and endemic are frequently applied. A disease is epidemic when it occurs in large numbers within a limited time. The occurrence of an epidemic implies the introduction in considerable quantity of an infective agent, and in such a case the disease will mostly be infectious. On the other hand, a disease is endemic when it is habitually prevalent in a particular locality. In that case the disease is most likely to be miasmatic in its origin.