A liquid preparation of treated disease-producing microorganisms or their products used to stimulate an immune response in the body and so confer resistance to the disease (see immunization). Vaccines are administered orally or by injection (inoculation). They take the form of dead viruses or bacteria that can still act as antigens, live but weakened microorganisms (see attenuation), specially treated toxins, or antigenic extracts of the pathogenic microorganism. For recombinant DNA vaccines, such as the hepatitis B vaccine, the gene encoding a suitable antigen, such as a protein subunit, is isolated from the pathogen and inserted into the genome of a host cell, such as the bacterium E. coli or yeast (see gene cloning). These recombinant cells are cultured so that they produce multiple copies of the subunit antigen, which is extracted and purified as a vaccine. Another strategy under trial employs DNA vaccines, which essentially introduce the genes for specific pathogenic antigens into body cells, so that the individual’s own cells synthesize the antigens and display them on the cell surfaces. The pathogen’s genes are inserted into a plasmid for delivery into the body by various methods. In principle, these vaccines should evoke an immune response with no risk of disease, as well as being relatively easy to make. Animal tests have shown their effectiveness in stimulating immunity against a range of diseases, but their clinical effectiveness remains to be established.