Cancer vaccines are not just a dream for the future. Several FDA-approved vaccines are cancer prevention vaccines. The hepatitis B vaccine and the human papillomavirus (HPV) vaccines prevent infection with cancer-causing viruses. By preventing the viruses from infecting body cells, these vaccines block the process that might eventually result in runaway cancer cell growth and damage to the body. The challenge for researchers is to use the model of the immune response to viral infection of cells to develop vaccines for cancers not caused by viruses. Just as the immune system constantly works to protect the body from harmful viruses and bacteria, it also plays a vital role in protecting the body from cancer. Many cancerous cells express markers called antigens that act as targets for the immune system. Today, the researchers are devising vaccines they hope will trigger the immune system to attack cancer cells reliably and effectively.