Guatemala City ? One of the Guatemalan twins who were born fused at the skull and separated by U.S. doctors is in critical condition, suffering from a brain infection, doctors said Friday.

Maria Teresa Quiej Alvarez was hospitalized last week after doctors discovered that a valve inserted in her skull to keep fluid from putting pressure on her brain had become infected.

Guatemalan twins Maria Teresa Quiej Alvarez, left, and Maria de Jesus and are held by their parents in this Jan. 22 photo. Doctors Friday said Maria Teresa had been in critical condition for 10 days, suffering from a cerebral infection.

“(Maria) Teresa is fighting to survive, and we’re fighting along with her,” said Ludwig Ovalle, medical director of Guatemala’s Pediatric Foundation.

Maria Teresa and her sister Maria de Jesus captured the world’s imagination when a team of U.S. doctors separated the pair in a 23-hour operation Aug. 6 at Mattel Children’s Hospital in Los Angeles.

The twins, now 21 months old, returned Jan. 13 to Guatemala City and have lived in a house built for them by the pediatric foundation.

But doctors discovered the problem with the valve, removed it and inserted a new device to drain fluid that could otherwise put too much pressure on the girl’s brain, possibly causing death, Ovalle said.

Ovalle also revealed the other twin had been hospitalized in March for a skin infection in her surgery scar. He said the infection was successfully treated with antibiotics.