When bad viruses do good work.

• Position: Vaccination

The polio vaccine was a medical triumph, singlehandedly decreasing the number of cases in the world from more than 350,000 in 1988 to around 400 today. Now, surgeons at Duke University, Durham, N.C., are using the once universally feared virus to target another disease--cancer.

One of the big problems with cancer is that the body does not realize it is dangerous. For most infections, the immune system learns to recognize what an infected cell looks like. Cells infected by a virus will display protein markers which alert the immune system that they are infected. Invading bacteria also display similar markers, aiding the immune system in finding and targeting them. In cancer, however, a person's cells appear just like they always do, even while they are multiplying uncontrollably. Your immune system has no way to distinguish these multiplying cells from healthy cells, and so it does not know to attack the tumor.

If a virus could be made to attack tumor cells only, then infecting tumors with such a virus could help the immune system clear away tumors--whether benign or malignant. It turns out the polio virus can do this. Because tumor cells are growing and multiplying rapidly, they look a lot like muscle cells in a healthy, growing...