1b). The back-projections suggest some evidence of an increase in HIV incidence in the late 1990s, but a plateau at around 40 HIV infections per year in the 2000s. The models estimate that a total of 1050 people were infected with HIV solely through IDU in Australia to the end of 2006, of whom 12% (95% CI 9%, 15%) are estimated to have remained undiagnosed (Table 2). The number of new HIV diagnoses for which exposure to HIV was attributed to heterosexual contact increased from 775 in 1997–2001 to 914 in 2002–2006, accounting

for 20% of the total Natural Product Library purchase HIV diagnoses (Annual Surveillance Report, 2007). Consistent with these results, back-projec-tion analyses suggest steady increases in new infections attributed to heterosexual HDAC inhibitor exposure to HIV (men and women) since the mid-1990s (Fig. 1c and d, respectively). The model estimates that a total of 1492 men and 1119 women were infected through heterosexual exposure, of whom 23% (95% CI 21%, 25%) and 22% (95%

CI 19%, 25%), respectively, are yet to be diagnosed with HIV infection (Table 2). In the absence of accurate tests for biological markers that can be used to determine the duration of infection in individuals, it is important to use all data available to estimate trends in HIV incidence over time. One of the advantages of our model for estimating HIV incidence is its ability to utilize the long history of HIV and Cyclin-dependent kinase 3 AIDS surveillance data while adjusting for changes in ‘testing behaviours’. AIDS surveillance data

were only used in the analysis for HIV incidence until 1987, just prior to the first antiretroviral drug becoming available. Overall, our results suggest that recent increases in HIV diagnoses in MSM in Australia do reflect an increasing trend in underlying HIV incidence over recent years. Similar increases in HIV diagnoses have been seen in MSM in virtually all developed countries [9]. Deterministic mathematical models suggest that reported increases in unprotected anal intercourse in MSM, and importantly increases in other sexually transmissible infections acting as co-factors for HIV transmission, can explain increases in HIV incidence in Australia [10]. According to our results, the rate of HIV transmission through IDU is currently relatively flat in Australia after an increase in incidence during the late 1990s. The increase in HIV incidence in the late 1990s coincided with an increase in the number of injecting drug users, and with an increase in the incidence of hepatitis C virus (HCV) infection [11]. It is widely acknowledge that, since 2001 in Australia, there has been a reduction in the heroin supply, resulting in some reduction in IDU, and also an estimated decline in HCV incidence [12].