The term cancer vaccines encompasses 2 different types of vaccines. Prophylactic vaccines block infection by viruses that can alter host DNA and result in cancer. The hepatitis B vaccine and the human papillomavirus vaccines are examples of prophylactic vaccines that can prevent cancer from developing. More recently, therapeutic vaccines have been developed and used as adjunctive therapy in patients who have already been diagnosed with cancer. Therapeutic vaccines stimulate the host’s immune system to recognize cancer cells as foreign and to attack them. Most of the therapeutic vaccines being studied are used in combination with other forms of cancer therapy.