• There are, to oversimplify, two kinds of immune response. The first is the antibody - 'humoral' - response. This is what is generated by gp120. Antibodies are little free-floating proteins that stick to invading viruses and bacteria, either killing them directly, or 'tagging' them for later destruction by immune cells. If HIV is an invading army, the antibodies are the resistance.
• The second kind is the CD8 - 'cellular' - response. CD8 cells are also called T-suppressor cells. Or, to use the term immunologists prefer, cytotoxic T-lymphocytes (CTLs). When the body's cells are infected with any virus, they 'display' bits of the virus, dotted on their surface. The CTLs are the counter-insurgents (to continue the war analogy) - they recognise these 'traitor' cells, lock on to them and kill them.

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Cellular vaccines

• So the next strategy is to wrap bits of HIV inside the shell of a harmless but infectious virus or microbe. They enter cells - which then display the HIV components on their surface, generating that CTL response. Adenoviruses (which sometimes cause colds), canarypox virus, and even a harmless version of the food-poisoning Salmonella bacterium have been used.
• You could call this entering of cells 'infection'. But it's one by the completely inactivated shell of a virus, which can't reproduce or threaten health in any way.
• However, as vaccines are designed to mimic natural infection, an effective HIV vaccine may mean that the recipient will test 'HIV positive'. It doesn't mean they will have an HIV infection - just an imitation of one, and more sophisticated diagnostic tests will be able to tell the difference.