Studies of HIV/sexually transmitted diseases (STDs) in China have paid little attention to community characteristics that may be conducive to HIV/STD risky behaviours and to the spread of the epidemics. Using data from a community survey and employing both correlation and multiple regression analysis, this paper examines the county-level determinants of the prevalence of HIV/STD risky behaviours and infections. The results suggest that community social and behavioural norms predict significantly the prevalence of illicit drugs and commercial sex and that the prevalence of the two HIV/STD risky behaviours reinforces each other. The analysis also shows that migration significantly increases prevalence of HIV and STDs, as does prevalence of injecting drug use and commercial sex. Overall, HIV and STDs are found not to be diseases of poverty but more likely byproducts of social and behaviour changes associated with development and urbanization.