In this chapter, using the specific case of bacteriophage viruses (literally: bacteria-eating viruses) and their bacterial hosts, I explore how taking into account this relational dimension of biological entities can allow us to imagine new therapeutic assemblages. Since their discovery at the beginning of the twentieth century, phages have indeed been used to treat bacterial infections. Although they were neglected in the second half of the twentieth century, notably due to the discovery and then massive production of antibiotics in the 1940s, there has been growing interest in their use since the early 2000s, due to the rise in bacterial resistance to antibiotics.