Primary – preventing a disease before it occurs. For example, reducing the chances that young people will take up smoking by providing school-based tobacco prevention programs. Secondary – preventing a worsening or future occurrence of a disease after evidence of the disease has already been found. An example would be a doctor removing a pre-cancerous skin lesion before it becomes cancerous and spreads. Tertiary – means treatment for an ongoing disease. An example would be treatment designed to reduce the spread of cancer metastases. [Adapted from CIHR]