Veterinary science draws on different traditions for knowing and acting, and mobilises different kinds of materials and techniques. This article explores these differences and their tensions for the diagnosis of foot and mouth disease in the UK in 2001. It shows that when they talk of foot and mouth disease, different veterinary traditions refer to the different objects. The clinic looks for deviances in animals, the laboratory detects the presence or otherwise of virus, while epidemiology focuses on patterns of transmission in populations. Despite the fact that they use the same word, clinic, lab and epidemiology are each involved with their own specific ontological variant of ‘the’ disease. At the same time other figures and configurations shift with the disease. This means that it is not possible to map different versions of foot and mouth disease onto a background of shared co-ordinates. So in 2001 clinic, lab and epidemiology diagnosed foot and mouth disease mobilising different kinds of materials, the entities inhabiting these practices had different qualities and they operated in different ways. Even time lines and spatial relations changed. Such differences are usually treated as a matter of perspective: it is assumed that everyone is looking at a single world. The article challenges this assumption by arguing that different veterinary traditions draw upon and contribute to different worlds in the plural. This shift makes it easier to explore the strengths of these worlds, their drawbacks and their limitations.