IMMUNITY, in medicine the name given to a condition of the living body in which it is safe from the attacks of par ticular infectious germs, or certan pois ons. Although the phenomena of im munity occur in cases of several vege table and animal poisons (ricin, and snake venom for instances) not asso ciated with disease, and are, hence, not strictly limited to the infectious diseases, they are of chief prophylactic and thera peutic interest in connection with the latter group of maladies. In a region where yellow fever abounds, certain in dividuals remain healthy, while their neighbors, fall victims to the fever. These persons are spoken of as im munes. Their immunity may be due to a natural insusceptibility, or may have resulted from a preceding attack of yel low fever. One attack of many of the infectious diseases confers immunity from any further attacks by the same germ. This fact, long ago noted by the medical practitioner (and utilized for the prevention of smallpox in China several hundred years ago), is now being applied in prevention and curative work in the case of many common infectious dis eases—typhoid fever, cholera, plague, cerebrospinal meningitis, etc. The im munity conferred upon person by an at tack of disease is called active; that resulting from an artificial treatment with vaccines or sera, passive. The remedies used in producing artificial im munity are called vaccines, antitoxins and toxins. Vaccines usually consist of living or dead bacteria or their poisons (toxins) ; sometimes the living organisms are weakened or attenuated by heat, g-rowth on special culture media, passage through other animals, etc., before use in
the human body. The injection of vac cines, in small, frequently repeated doses, stimulates the normal protective agencies of the body. If the special disease germs, for which the vaccine is intended, are present in the body, in other words if the man already has the disease, the vaccine serves as an aid to the regular body forces, in raising a defensive army. If the disease is not present, the vaccine builds up a condition of immunity, so that an attack by the germ, if it occur at a later period, can be warded off. Vaccines have been more successful as preventives than as cures. The immu nity produced may be permanent and lasting or merely temporary, a few weeks or months. Antitoxins are substances which act as antidotes to the toxins pro duced by disease germs. They are com monly manufactured by repeatedly in jecting in an animal (horse, goat, dog, etc.), with increasingly virulent doses of the organism or its poisons against which an antitoxin is desired. The animal's blood finally becomes highly re sistant to this particular organism, and it can withstand without harm many times the dose of germs fatal to a similar animal not so artificially immunized. A portion of the blood of the immunized animal is drawn, and the serum obtained from this constitutes the essential ele ments of the commercial antitoxin.