Abstract:  Viruses, and particularly phage that infect bacteria, are the most abundance and diversity life forms on the planet. Given their success throughout the biosphere, it is expected that phage are essential members of the animal and plant holobionts. We have shown that phage form a bacterial selective, adaptive immune system that helps protect the mucosal surfaces of animals and establish the microbiome. Additionally, phage are actively transported across epithelial layers and provide a systemic protection against bacteria. These two findings strongly suggest that phage formed the first acquired immune system and they remain important in extant animal immunology.