Long-standing patterns of drug injecting in mainly developed countries have been joined by the introduction and expansion of drug injecting in developing countries, often followed quickly by outbreaks of human immunodeficiency virus (HIV) infection. Drug injecting has been identified in 80 countries and HIV infection in 52 of these. Given the continued recruitment of new injectors where injecting is endemic, the diffusion of drug injecting in countries where the practice was formerly rare and the potential for this diffusion to occur, HIV prevention must also consider interventions that discourage drug injecting. Particularly at risk are countries in drug-producing regions and along drug transit routes in Africa, south-east and south-west Asia and South America. Injecting can spread in less time than it takes to introduce HIV prevention activities. The task of the public health system will be to find a balance between activities targeted at helping current injectors change their behaviour and activities targeted at discouraging the adoption of drug injecting.