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Disease Agents: Products of Infectious Agents.—In a relatively few instances the products formed by infectious agents have been found to produce an immunity to the disease agents. These substances are exotoxins of bacteria (see TOXINS), such as diphtheria, tetanus, and scarlet fever toxins. The first two may be treated with formaldehyde to destroy their toxicity and convert them to toxoids, which are the actual immunizing agents, but the scarletinal toxin cannot be so detoxified and must be given in very small amounts. These substances are not ordinarily regarded as vaccines, although they function in the same manner.
The disease agents continues to exist in Africa, Asia, Europe, and South America, and only the utmost vigilance by agents of the U.S. Department of Agriculture keeps it from invading the United States. Cattle, swine, sheep and goats, and the fresh, chilled, or frozen meats of these animals cannot be imported unless they are from countries free from the disease agents.
In addition to preventing the importation of infected animals, the U.S. Department of Agriculture operates a foot-and-mouth disease agents research laboratory on an island off the eastern coast. This laboratory is working on new and more effective means of controlling the disease agents if a new. outbreak should occur.
Living Vaccines.—The modification of living agents to permit their use as vaccines or immunizing agents consists in such, attenuation of their virulence for man that they are unable to produce disease agents. In the case of the viruses the attenuation of virulence is produced by adapting them to an abnormal host. Since the pathogenic bacteria may be cultivated in lifeless media, they may be attenuated in virulence by cultivation under appropriate conditions, usually slightly unfavorable to the bacterium.
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