JOHANNESBURG (AP) ― Nelson Mandela was back in the hospital for the third time in four months Thursday, and the 94-year-old former South African president was reported to be responding well to treatment for a chronic lung infection.

South Africa’s presidency said that doctors were acting with extreme caution because of the advanced age of the anti-apartheid leader, who has become increasingly frail in recent years.

The Nobel Peace Prize laureate was admitted just before midnight to a hospital in Pretoria, the South African capital. He has been particularly vulnerable to respiratory problems since contracting tuberculosis during his 27-year imprisonment for fighting white racist rule in his country.

“The doctors advise that former President Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection,” the presidency said in a statement. “He remains under treatment and observation in hospital.”

Mandela, who became South Africa’s first black president in 1994, is a revered figure in his homeland, which has named buildings and other places after him and uses his image on national bank notes.

“I’m so sorry. I’m sad,” Obed Mokwana, a Johannesburg resident, said after hearing that Mandela was back in the hospital. “I just try to pray all the time. He must come very strong again.”

In December, Mandela spent three weeks in a hospital in Pretoria, where he was treated for a lung infection and had a procedure to remove gallstones.

Earlier this month, he was hospitalized overnight for what authorities said was a successful scheduled medical test.