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.