Mucous membranes lining the mouth and other cavities of the body would prove favorable sites for the growth of bacteria if the mucus secreted by them were not frequently removed. The mouth of a healthy person may contain bacteria of many kinds, but the saliva has a slight disinfectant power and serves as a constant wash to the membranes. The normal gastric (stomach) juice is decidedly unfavorable to the growth of bacteria,
although it does not always kill them; they often pass through the stomach and are found in large numbers in the intestines. Other bodily secretions, such as the tears and perspiration, tend to discourage bacterial growth.
Tissues of the body vary greatly in their power to resist invading germs, so that the route by which germs enter influences the severity of their effects. Typhoid bacilli and the spirilla of Asiatic cholera when taken with food or water produce far more serious disturbances than when injected under the skin; infections from pus germs through an abrasion of the skin may result in a slight local disturbance, while the same amount introduced into a deeper wound might cause a fatal infection. Certain germs nourish in certain tissues only; even tuberculosis, which attacks practically all tissues, has its favorite locations.
Immunity.
—In addition to its mechanical defenses against disease, the body shows a varying degree of immunity, or the power possessed by living organisms to resist infections. Immunity or resistance is the opposite of susceptibility. It is exceedingly variable, being greater or less in different people and under different conditions, but the exact ways in which it is brought about are still in many cases far from clear.
Immunity may be natural or acquired. By
natural immunity is meant an inherited characteristic by which all individuals of a species are immune to a certain disease. The natural immunity of certain species of animals to the diseases of other animals is well known. Man is immune to many diseases of lower animals, and they in turn are immune to many diseases of man. Cattle, for instance, are immune to typhoid and yellow fever, while man shows high resistance to rinderpest and Texas fever; both, however, are susceptible to tuberculosis, to which goats are immune. There are all gradations of immunity within the same species. Moreover, certain individuals have a personal immunity against diseases to which others of the same race or species are susceptible.
Immunity may be acquired in several ways. It is commonly known that one attack of certain communicable diseases renders the individual immune for a varying length of time, and sometimes for life. Among these diseases are smallpox, measles, whooping-cough, scarlet fever, infantile paralysis, typhoid fever, chicken-pox, and mumps; erysipelas and pneumonia on the other hand appear to diminish resistance and to leave a person more susceptible to later attacks.
Again, in some cases immunity may be artificially acquired by introducing certain substances
into the body to increase its resistance. Examples of this method include the use of antitoxin as a protection against diphtheria, of sera in pneumonia and other infections, and vaccination against smallpox and typhoid fever whereby a slight form of the disease is artificially induced. Laboratory research goes on constantly, and doubtless many more substances will eventually be discovered that will reduce human misery as vaccines and antitoxin have already reduced it.