An unrelated virus or a bacterium (called a “vector”), itself harmless, can be used to transport a gene vaccine into the body. Another way to transport a vaccine is to simply immunize with a circle of DNA coding for an antigen. In a key study, mice were given DNA that coded for a protein of influenza virus. When later given a different strain of live influenza virus, the mice survived what would have been a fatal infection. In clinical studies, DNA vaccines have produced antibodies and cell responses against the agents that cause AIDS, hepatitis, malaria, and other serious infections. The immune responses have been somewhat weak, however. Large-scale trials are just now getting under way to test DNA vaccines in populations at risk for serious infectious diseases. In time, gene vaccines may also be used to treat certain types of cancer, allergies, and autoimmune diseases (in which the body attacks its own cells). In the patient trials done to date, gene vaccines have been well tolerated and safe.