Making vaccines by simply weakening the virus that causes AIDS has failed because the virus mutates a hundred times faster than even the fast-mutating flu virus. In Dr. Picker’s vaccine, S.I.V. genes are fused to those of another virus, the cytomegalovirus. (The name means “big cell,” and it is in the herpes family but different from its relatives that cause lip and genital sores, chickenpox and shingles.)

H.I.V. fusion has been tried with adenoviruses and others, but cytomegalovirus seems to work better. It’s not entirely clear why, but one theory is that cytomegalovirus has a very long history of infecting primates — so much so that 100 percent of monkeys and about 80 percent of humans get it in their lifetimes.

Therefore, we primates have adapted to it. Although the virus can be lethal to fetuses and to those with immune systems suppressed by AIDS or transplant drugs, in most victims it causes no symptoms.

The body responds to cytomegalovirus more slowly and calmly than it does, for example, to a flu.

As in any infection, the thymus gland generates new white blood cells called T cells — in this case, CD8 hunter-killer cells — primed to target the specific virus. But in the case of Dr. Picker’s vaccine, those cells stay in an unusual “half-alert” state. A full-blown immune response eventually exhausts itself, and can even be dangerous. For example, the rare humans who catch H5N1 bird flu often die of the immune response itself; they drown in the flood of CD8s and other would-be saviors pouring into the lung tissue, spoiling for a fight.

That “half-alert” state is the “Kafkaesque” element: unactivated CD8s wander around aimlessly, while fully activated ones behave like storm troopers. But the half-activated CD8s persist in tissues, eliminating their targets quietly without triggering inflammation or even a mild fever.

When S.I.V. genes are fused to the cytomegalovirus spine, the CD8s kill S.I.V.-infected cells too.

Since it protected only some monkeys, the new technique might be best used in combination approaches. For example, Dr. Fauci said, it could be given with a vaccine that generates antibodies against H.I.V. “and maybe eliminate the cells that sneak past the antibody shield.”