It is recognised that people movement can increase potential risk of HIV transmission. In recent years, mobile populations moving across national borders have become a focus for HIV/AIDS prevention campaigns. These programs generally target border "hot zones" that produce high levels of HIV vulnerability due to the degree of mobility and the risk behaviours fostered by these marginal environments. However, high degrees of movement and social exploitation need not be the only criteria for borders to exacerbate HIV vulnerability. The types of social interactions promoted by mobility take many forms. In this paper we consider a border zone between Thailand and Laos to show that the links between movement and HIV vulnerability are not confined to stereotypical instances of coercion and exploitation. Rather we demonstrate that HIV risk in this area is a product of both a sense of community and a sense of difference that together foster a range of interactions based on mobility back and forth across the border. As HIV/AIDS prevention programs increasingly control forms of sexual interaction, the border provides a practical and symbolic opportunity to establish new forms of sexual relationship falling outside these constraints. This tendency to move outside bounds is not limited to border areas but has implications for prevention programs everywhere.