The introduction of antiretroviral therapy has led to a dramatic improvement in rates of morbidity and mortality for individuals infected with HIV-1. However, the emergence of drug-resistant strains has proved a major obstacle to achieving successful treatment regimens. In particular, the phenomenon of primary or transmitted resistance, when an individual is infected by a strain of HIV-1 already resistant to one or more drugs, has emerged as a potential threat to the success of antiretroviral therapy. The increased use of antiretroviral agents in both developed and developing countries has led to an increase in the incidence of drug resistance and subsequently a large pool of resistant viruses becoming available to establish new infections. Transmission of drug-resistant viruses occurs irrespective of the route of infection [1–3]. However, when assessing the epidemiology of primary resistance, one should be careful to exclude the possibility of undisclosed drug exposure, particularly in patients who may have lived in more than one country and for whom previous medical records may be unavailable for consultation.
