Drawing on the example of bovine tuberculosis (bTb), this paper examines the geographies of animal health promotion. Using theories from the sociology of health, the paper outlines how the spatial practices of animal health promotion have had adverse policy consequences – what the paper refers to as an ‘ecological paradox’. Analysis of ethnographic interviews with 61 farmers in England and Wales provides a range of reasons why farmers do and do not implement biosecurity. Drawing on the concept of lay epidemiology and ideas of ‘the candidate’– that is, the terms by which someone/thing is most likely to suffer from a particular illness – the paper shows how farmers construct farmers, cattle and badgers as likely to be a candidate for bTb; and how aspects of luck and fatalism are significant elements of candidature. These effects are traced to a clash of spatial practice within the different knowledge articulated by official attempts to promote animal health and farmers’ understandings. In failing to consider these cultural understandings of disease, the paper argues that the state's attempts to promote animal health have served to reinforce the explanatory power of candidacy and traditional understandings of bTb, thereby overriding attempts to promote biosecurity. The resulting negative consequences for badgers, cattle and farmers are defined as the ecological paradox.