The presidency said Mandela was responding well to treatment and the medical team was acting with extreme caution because of the age of the anti-apartheid leader, who has become increasingly frail in recent years.
The Nobel laureate was admitted late on Wednesday to a hospital in Pretoria, the South African capital, in the latest in a series of such trips since last year. He has been particularly vulnerable to respiratory problems.
Mandela, who became South Africa’s first black president in 1994, contracted 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 is a revered figure in South Africa, which has honoured his legacy of reconciliation by naming buildings and other places after him and printing his image on national banknotes.
“I’m so sorry. I’m sad,” said Obed Mokwana, a Johannesburg resident, after hearing that Mandela was back in a hospital.
“I just try to pray all the time. He must come very strong again.”
Earlier this month, Mandela spent a night in a hospital for what officials said was a scheduled test.
Presidential spokesman Mac Maharaj, referring to Mandela by his clan name “Madiba”, said that Wednesday’s trip was not for previously planned treatment.
“No, this wasn’t scheduled, as you will appreciate the doctors do work with a great sense of caution when they are treating Madiba and take into account his age,” he said.
“And so when they found that this lung infection had reoccurred, they decided to have him immediately hospitalised so that he can receive the best treatment.”
In December, Mandela spent three weeks in a hospital in Pretoria, where he was treated for a lung infection and underwent a procedure to remove gallstones.
Speaking from the coastal city of Durban, before it emerged that Mandela was responding well to hospital treatment, Maharaj acknowledged there was cause for worry, but said the specialists treating Mandela were very competent.
“With Madiba and a person of his age, there always has to be concern and therefore the doctors, I think we need to appreciate, will prefer to work on the side of caution rather than taking any risks.”
Maharaj said there had been a global outpouring of messages expressing concern for Mandela’s health.
President Jacob Zuma wished Mandela a speedy recovery. “We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts.
“We have full confidence in the medical team and know that they will do everything possible to ensure recovery,” his office quoted him as saying.
Mandela was released by the apartheid government in 1990. He became South Africa’s first democratically- elected president in 1994, under the banner of the African National Congress and helped to negotiate a relatively peaceful end to apartheid, despite fears of much greater bloodshed. He served one five-year term as president before retiring.