It is hypothesized that large classes of facts concerning host-parasite relationships in both the animal and plant kingdoms can be understood in terms of the biochemistry of the host and the nutrition of the parasite without recourse to special agents such as antibodies, antibiotic substances, phagocytes, barriers, enzymes, and toxins. The hypothesis is made reasonable by what is known of the positive and negative effects of various nutrilites upon the growth of organisms and by what is known about the presence of these same substances within host organisms. Some facts which give circumstantial support and a few facts which give direct support to the idea are presented. Mention is made of possible lines of investigation to be followed and of possible difficulties to be encountered.