The host is an organism which affords lodgment and sustenance to parasite and commensal organisms. The invading organisms may be beneficial, they may be harmful or they may produce no effect. Parasites Parasites are those living organisms, either plant or animal, which live in, on, or with some other living organism, known as the host, from which they obtain their food, shelter and other advantages. They are of two classes—ectoparasites and endoparasites—and may be either plant or animal organisms. An ectoparasite is one that lives upon the outside of another living organism known as the host. The endoparasite is one that lives within the body of the host. Commensal A commensal is an organism which lives in or on another organism and shares the shelter and partakes of the same food as the host, but is not in reality a parasite. It is a close association of two organisms for the benefit of one or both. Antibiosis This is a form of parasitism which is harmful to either host or parasite or to both. By this process of antibiosis water may be purified of many of the pathogenic germs. It is also maintained that in many instances there is but one major condition in the body at one time as a result of this process. Symbiosis In this form of parasitism the organisms are dissimilar, but neither is harmful to the other while the state is beneficial to one or both. Symbiosis will result in one condition becoming conducive to another condition as, for example, presence of certain microörganisms favors the development of tetanus. Germs are sometimes cultivated in symbiosis with other germs. This has been done with the germs that are found in leprosy. Antibodies Antibodies are bodies as the term would indicate, but refers to specific properties of the blood supposed to be produced by antigens and thought to accomplish a certain degree of immunity. Antigens Antigens are the substances which produce the antibodies in the blood and body fluids. These are merely terms used to designate the mechanism of immunity. Saprophytes Saprophytes are microscopic organisms which feed upon dead organic matter. A saprophyte is a vegetable organism. Obligate Saprophytes Obligate saprophytes are saprophytes which can not be made to develop under any circumstances in living tissue, but will develop in dead tissue in which they produce toxins. The diphtheria and tetanus bacilli develop in a localized area of dead tissue and produce the toxins peculiar to this condition. Septicemia Septicemia is a condition wherein so-called pathogenic bacteria and their accompanying poisons are present in the blood. These bacteria may develop within the blood or they may be carried into the blood from some local point where they are developing due to the condition of the tissues as a result of interference with transmission of mental impulses. Toxemia