Blood-borne viruses are viruses that some people carry in their blood and can be spread from one person to another. Those infected with a blood-borne virus may show little or no symptoms of serious disease, but other infected people may be severely ill. Our research focuses on reducing the transmission of blood-borne viruses among people who inject drugs. People who inject drugs are at high risk of contracting blood-borne viruses such as HIV and Hepatitis C due to sharing dirty needles. Much of our work involves modelling the impact of different interventions to reduce the transmission of HIV and Hepatitis C among this population group, and assessing the cost-effectiveness of these interventions. Our work has informed HIV and Hepatitis C elimination strategies in the UK and other countries, international treatment guidelines and the World Health Organization's elimination strategies.