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.