The response many gay men took to the HIV epidemic in the United States was largely informal, especially given distant state and federal governments. The condom code, a set of informal norms that encouraged the use of condoms, is one instance of this informal response, which was wholly uncoordinated. Yet, it is not clear why these informal norms emerged or why they have since eroded. This paper explores how gay men in particular generated expectations and normative beliefs regarding condom usage, (...) which helped to establish the condom code as an informal norm. Furthermore, the erosion of the condom code is viewed as a result of changing expectations, which change as bio-medical means of HIV treatment and prevention develop and as online and digital communities facilitate serosorting, all of which provide alternatives to condoms as a means of prevention and their associated informal norms. Future HIV prevention campaigns should recognize the extent to which informal norms coordinate and encourage preventative behavior, as well as how beliefs and expectations alter the informal norms people adopt. (shrink)