Decades into the HIV epidemic, we know surprisingly little about the relative burden of HIV among sex workers. UNAIDS estimates that less than 50% of sex workers have access to HIV prevention programmes worldwide.1 HIV prevalence among sex workers is highly heterogeneous both across and within regions. In addition to individual and biological drivers (eg, the epidemic structure, co-infection with other sexually transmitted infections [STIs], and antiretroviral therapy [ART] coverage), structural features continue to play a crucial part in shaping risk of infection among sex workers and their clients, including work environment, violence, stigma, and the legal, cultural, and political contexts of sex work.2 and 3 In The Lancet Infectious Diseases, Stefan Baral and colleagues4 report a systematic review and meta-analysis of HIV burden among female sex workers in 50 low-income and middle-income countries. Female sex workers had an overall increased odds of HIV infection (odds ratio 13·5, 95% CI 10·0–18·1) relative to the general female population of reproductive age, with the highest odds in Asia (29·2, 95% CI 22·2–38·4) and lowest in Latin America and the Caribbean (12·0, 95% 7·3–19·7). Baral and colleagues show substantially high HIV prevalence levels across geographical regions and epidemic structures, including both concentrated epidemics and mature, generalised epidemics, as in sub-Saharan Africa where the pooled HIV prevalence was 36·9% (95% CI 36·2–37·5). The heavy HIV burden points to the crucial need for urgent scale-up of comprehensive initiatives simultaneously targeting HIV prevention, ART access, and care among female sex workers, especially in view of the established role of treatment as prevention. Of concern, two-thirds of countries have no data on HIV burden in this group. As Baral and colleagues suggest, the dearth of data can be attributed in large part to the same structural conditions that increase risk of HIV and prevent engagement in interventions among female sex workers, including criminalised legal and policy environments, violence, stigma, and restrictive funding policies.5 and 6 All too often moral debates dominate the public health response among sex workers, and science continues to take a backseat to punitive approaches and raid and rescue operations aimed at eliminating sex work. The US antiprostitution pledge enacted in 2003 continues to prevent non-governmental organisations from accessing funds from the US Presidents Emergency Plan for AIDS Relief if they did not oppose sex work. Evidence suggests that in settings where sex work is criminalised or heavily sanctioned (eg, mandatory testing), sex workers are pushed outside the public health system.6 and 7 The common threat of violence towards sex workers by clients and third parties (eg, police, exploitative managers, pimps) are associated with increased risk for HIV infection.8 Enforcement efforts (eg, police crackdowns, raids), punitive sanctions, and the threat of violence can deter sex workers from accessing HIV prevention services, constrain their ability to negotiate condom use, and prevent the roll-out and rigorous assessment of HIV interventions.6 Individual country and programme successes have made important progress regionally, but for the most part have not been met with global backing from international and governmental bodies, and face the same structural barriers to large-scale implementation and rigorous assessment. A systematic review9 of HIV and STI intervention studies among female sex workers in low-resource settings showed significant use of multicomponent and structural interventions (policy change and sex-work collectivisation), with only modest effects reported for behavioural interventions alone. Multipronged HIV-prevention strategies have been adopted in several countries (eg, safer sex-work environments),2 with the most well documented structural HIV interventions being those led by sex workers in India (Songachi and Avahan models).10 and 11 The results stand as an important marker of the sheer scale of the HIV epidemic among female sex workers worldwide, and a call to action, both for investment in science and scale-up of evidence-based HIV-prevention interventions. With efforts to better characterise the epidemic in sex work and the response to it, research and interventions among the male client population and other subpopulations of sex workers (eg, male and transgender sex workers, migrant workers) are also crucial. As the epidemic matures in many settings, with some countries already reporting over 50% of sex workers living with HIV, comprehensive initiatives simultaneously targeting HIV prevention, ART access, and care are increasingly vital. As highlighted in a recent report by the UNAIDS advisory on sex work and HIV,12 removal of structural barriers (eg, criminalised laws and policies, violence) remains a necessary precondition to an effective HIV response in sex work worldwide.