Immune responses and mucosal surfaces of corals and humans are strikingly similar; this is interesting because these two animal groups are separated by over 500 million years of evolution. Here we propose the Bacteriophage Adherence to Mucus (BAM) model of specific innate immunity, a defense applicable to all metazoans. The model links hypervariable phage capsid decoration proteins with the adherence of phage to mucus and it's consequent reduced bacterial pathogenesis of underlying host epithelium. The relationships delineated make the world's most abundant biological entities central to the metazoan immune system. In so doing, they advance our understanding of immunology and the microbial ecology of a key metazoan-associated environments, and will open new directions in immunological engineering.