There may be no better example of what is meant by preventive medicine than the strategy of Vaccination. A healthy person is given a tiny taste of a virus—flu or polio, say—that’s too weak to cause illness but just enough to introduce the body to the pathogen. If the virus later shows up for real, the immune system is primed and waiting for it.

That’s close to how a cancer vaccine works, but not precisely. Most experts see cancer vaccines as a hybrid of treatment and prevention. While it’s true that the U. S. Food and Drug Administration has ap-proved vaccines against cervical and liver cancer, both are designed to fight the viruses most responsible for causing the disease, as opposed to targeting cancer itself—human papilloma virus (HPV:人乳头瘤病毒) in the case of cervical cancer and hepatitis B in the case of liver tumors.

Using vaccines to prevent nonviral cancers in someone who is disease-free is a whole different matter. For one thing, it’s much more difficult to determine a person’s chance of developing a particular type of cancer than it is to determine the likelihood of being exposed to, say, the influenza virus or chicken pox. What passes for “exposure” in the case of nonviral cancers is a combination of genes and environment and a range of other X factors that can vary from person to person. How do you vaccinate against your family legacy of breast cancer or your constant exposure to secondhand cigarette smoke?

But that doesn’t mean the immune system can’t be exploited in a different way. Cancer vaccines would ideally be used in patients whose disease has already been diagnosed and treated with surgery, chemotherapy or radiation. They would then be immunized as a way to prevent the cancer from coming back and spreading. Such metastases are actually the leading cause of death from cancer. “The charm of working with the immune system is that we can use the body’s own defense mechanisms to possibly get to that last cancer cell or at least create a surveillance system that keeps that cancer under control,” says an oncologist.

Which of the following is true of vaccines against cancer?