Bacteria, like mammals, have to constantly defend themselves from viral attack. Like mammals, they use both innate and adaptive defense mechanisms. In this point of view we highlight the commonalities between defense systems of bacteria and mammals. Our focus is on the recently discovered bacterial adaptive immune system, the clustered regularly interspaced short palindromic repeats (CRISPR) and their associated proteins (Cas). We suggest that fundamental aspects of CRISPR/Cas immunity may be viewed in light of the vast accumulated knowledge on the mammalian immune system, and propose that further insights will be revealed by thorough comparison between the systems.