Sex workers have their perspective on HIV transmission, claiming that in general they are more similar than different from other people in HIV status and the practice of safe sex. Such an assertion of similarity goes against public and professional opinion that prostitution is a major vector in the spread of AIDS. Taking the sex workers' similarity claim seriously, this paper considers the conditions under which it would be valid. We focus on those factors that make a population more or less vulnerable to HIV and how they affect its spread into the sex work population. Data from New Zealand comparing men sex workers and other men who have sex with men is used to evaluate these ideas. Data partially support the hypothesis in that these two groups of men are similar with regard to their HIV status. We do find the sex workers to be different, however, in their being less likely to engage in safe sex practices. We provide an explanation for why this has not lead to their having a higher rate of seropositivity.