Towards a model of host-parasite relationships

The question asked in this article is: "what is a parasite?". Defining a pa rasite requires defining its host at the same time. A difficult question th erefore arises about host-parasite relationships. The object of general par asitology is in fact to study the relationship between a host and its paras ite. The initial question "what is a parasite?" has to be reformulated with in a conceptual framework, that of "relationship". This article is an attem pt to transpose into parasitology some concepts which have been profitable in the fields of physics and chemistry. The studies of R. Thom about the in dividual are also used. Concepts like that of field and principles like tha t of minimization are probably operational on a high level of organization. The description of host-parasite relationships could benefit from these co ncepts.