The article focuses on the activity of civil society organizations working in the prevention of STI in Portugal, in order to analyze how they perceive sex workers and design the social support programs addressed at them. The study is based on a qualitative analysis of the discursive production of privileged informants with relevant intervention in this area. The data we gathered allowed us to identify some obstacles opposing the access to the health care system and to HIV/Aids and STI prevention, namely the discriminatory discourses and practices towards these sex workers. The dichotomy vulnerability / threat tag the figure of sex workers and the public debate around sex work.