Such are a few of the infectious diseases which we can readily attribute to the presence of definite micro-organisms in respective cases. But strange as it may seem, the most typical of all infectious diseases, small-pox, scarlet fever, measles, and hydrophobia, have as yet not yielded up their secrets. This is possibly due to the minute size of the micro-organisms concerned, which make it beyond the power of the best microscope to demonstrate them. In this connection it has recently been shown by Roux and Nocard that in the case of the disease known as pleuro-pneumonia of cattle the causative agent is so very small as just to be barely visible. Again, it is quite possible that these diseases may be caused by living things we know nothing about, which may be quite dissimilar from the bacteria.
INFECTION—ITS PROCESSES AND RESULTS
In the foregoing list of diseases associated with specific bacteria, attention has been drawn to the common modes of infection, or, as they are technically called, “portals of entry,” and it now remains to touch upon the main factors, processes, and results following upon the entry into the body of such disease-producing microbes.
It is a well-known fact that the normal blood has of itself to a considerable extent the power of killing germs which may wander into it through various channels. Likewise the tissue cells of the body in general show similar action depending upon the different cell groups, state of health, general robustness, and period of life. The germ-killing power varies in different individuals, though each may be quite healthy. Considered as a whole, this power possessed by the body against germs is known as “general resistance.” And when by any means this power of resistance is lost or diminished, we run grave risks of incurring disease.
Granted a case of infection, let us now trace up briefly what occurs. Between the period when the bacteria gain a lodgment and that in which the disease assumes a noticeable form, the patient simply feels out of sorts. It is during this stage that the blood and tissues are deeply engaged in the attempt to repel the attacks of the invading microbes.
With varying speed the germs multiply throughout the body generally, or may be at first localized, or even, as in lockjaw, remain localized throughout the entire disease. Multiplying in the tissues, they generate in increasing amounts their noxious poisons, which soon cause profound changes throughout the body; the patient becomes decidedly ill, and shows now the signs of an unmistakable infection.
Does the body now give up the fight entirely? No; on the contrary, the white blood-cells, the wandering cells, and the cells of the tissues most affected still carry on an unequal fight. From the lymphatic glands and spleen, armies of white cells rush to the fray and attempt to eat up and destroy the foe, but possibly in vain; the disease runs its course, to end either in death or recovery.
How, then, in cases of recovery, are the microbes finally overcome?
This question involves many complex processes which at present are by no means thoroughly understood, but we will concern ourselves with the simple principles.
It has been previously mentioned that once the bacteria get a good foothold the body is subjected to the action of generated poisons, which are known as toxins. They give rise to such symptoms as loss of appetite, headache, fever, pains and aches, and even a state of stupor or unconsciousness. In addition to the active warfare of the white blood-cells, groups of cells throughout the body, after recovering from the first rude shock of the toxins, begin to tolerate their presence, then effect a change in the chemical constitution of the toxins, and finally elaborate substances which antagonize the toxins and destroy their action altogether, thus lending aid to the warrior cells, which at last overcome the invading microbes. Recovery is brought about, and a more or less permanent degree of immunity against the special form of disease ensues.