NEW YORK — A Los Angeles boy who was infected with the AIDS virus at birth apparently fought off the infection and is virus-free at age 5, astonishing his doctors.

Dr. Yvonne J. Bryson, a pediatrician and AIDS specialist at the UCLA School of Medicine in Los Angeles, said she believes it is the first carefully documented case of someone casting off all signs of infection.

Tests proved conclusively that the boy was infected for at least a month during the first two months of his life. Later examinations found no sign of the virus, Bryson said.

Doctors have no explanation.

“It’s like a miracle to me and a miracle to his mother. It’s every mother’s dream that their child won’t be infected,” Bryson said.

Bryson said she and her colleagues are studying the boy’s immune system for clues that could be used to stop AIDS infections in others.

“It just opens up a lot of things to look at. Before now, there was such skepticism about the possibility that this could occur,” said Bryson, whose findings appear Thursday in the New England Journal of Medicine.

The boy’s mother remains infected but does not yet have symptoms of AIDS, Bryson said. The boy and his mother were not identified.

Doctors cannot be certain the virus isn’t hiding somewhere in the boy’s body. But his continuing good health is additional evidence that the infection has disappeared.

Most children infected at birth get sick during the first three years of life, Bryson explained. If the boy were still infected, he would almost certainly be sick by now, she said.

“Our reaction first was that we must have made a mistake,” Bryson said. “I was skeptical, as other people would be.”

Bryson has since identified another child who appears to have accomplished the same improbable feat. Studies are now under way to confirm the second case, she said.

Dr. Gene Shearer of the National Cancer Institute said he has seen a few patients resist AIDS infection when exposed to the virus. But he said he hasn’t seen patients recover after becoming infected.

He said Bryson’s findings “are probably real.” The boy could have developed an immunity to the virus before birth, Shearer said.

Bryson warned AIDS patients, however, not to cling to the remote hope that their infections could disappear. “It’s probably a rare event, but it may be more common than we recognize,” she said.

She and her colleagues are trying to discover whether the boy’s immune system offered him special protection against infection. They might be able to harness that knowledge to protect others, Bryson said.

Bryson said the AIDS virus was isolated from the infant at 19 days and 51 days, showing he was infected on each occasion. Further analysis of the genetic material in the viruses showed that both virus samples were identical, meaning the results were unlikely to be due to error.