For some time, doctors have known that a small slice of humanity — less than 1 percent of the population — has a natural immunity to AIDS, thanks to a genetic quirk.

Now, the University of Minnesota has joined a global effort to find a way to extend that genetic immunity to the other 99 percent.

That's one of the goals of an extraordinary experiment that U doctors announced last week. They injected a young patient with blood cells from a donor with that rare genetic makeup, in an attempt to cure the boy of two illnesses, including HIV.

Scientists have known about the genetic mutation since the 1990s, said Dr. Timothy Schacker, an AIDS specialist at the university.

"We found a group of people who were repeatedly exposed (to HIV), and they were unable to get infected," he said.

Researchers discovered that the lucky ones shared a rare mutation on chromosome 3 that limited their ability to produce a protein called CCR5.

It turns out that most strains of the AIDS virus need that protein to infect a patient's cells. Without it, Schacker said, it's like a lock with no key; the virus can't get in.

The fortunate gene shows up most frequently in northern Europeans. About 1 in 100 Caucasians inherit it; but it's far more rare in other races.

The theory, Schacker said, is that the gene may be linked to the bubonic plague, which ravaged Europe in the 14th century. The same mutation may have protected people from that infection and been passed on by the plague's survivors.

"Sort of evolution in action," he said.

People born with two copies of the rare gene — one from each parent — have the strongest immunity to HIV. Since that discovery, scientists have been trying to figure out a way to capture that natural immunity and turn it into a treatment. But so far, there's been limited success.

"This is not a cookbook where we have all the answers," said Dr. John Wagner, a U transplant specialist. But, he said, it may be the best hope for his young patient at the university, and perhaps lead to a cure someday.