HIV/AIDS continues to be a major health issue in Canada, twenty-five years after the first cases were reported. About 58,000 Canadians, including 13,000 BC residents, are infected with HIV (the human immunodeficiency virus that leads to AIDS), and the incidence appears to be rising. A rough estimate sets the medical costs of caring for people with HIV/AIDS at more than $800 million a year. But rapid treatment advances make medical costs a moving target. Karissa Johnston is using the computer simulation model she developed in her earlier MSFHR-funded research to more accurately estimate the annual and lifetime medical costs of treating people infected with HIV. Johnston has designed a series of modules to measure the amount of HIV virus in peoples’ bloodstream (called the viral load) over their lifetime, their initiation and adherence to antiretroviral medications, their use of health services, and their survival time with different treatment regimes. As new treatments or data become available, individual modules can be updated without affecting the others. This information can help health care providers assess the costs and effectiveness of different treatment options. For example, antiretroviral medications successfully suppress viral load, reducing the risk of passing the infection during a sexual encounter. Even though the medications are costly, this tool will show if they ultimately result in costs savings due to a reduction in new infections.