Nelson Mandela receiving treatment for lung infection

CAPE TOWN, South Africa — Nelson Mandela, former president of South Africa, has a lung infection, his doctors say.

Mandela, 94, known in South Africa by his clan name "Madiba," is spending a fourth day at 1 Military Hospital in Pretoria, where he has been undergoing unspecified medical tests. 

"Doctors have concluded the tests, and these have revealed a recurrence of a previous lung infection, for which Madiba is receiving appropriate treatment and he is responding to the treatment," a Tuesday statement from President Jacob Zuma's office said.

No information has been given about when Mandela might be released from the hospital. He is said to be “comfortable” and not in any immediate danger.

Mandela's wife, Graca Machel, said in a 2009 interview that it was painful to see her husband aging. 

"I mean, this spirit and this sparkle, you see that somehow it's fading," Machel told eNews Channel Africa in an interview re-broadcast Monday.

"To see him aging, it's something also which pains you … You understand and you know it has to happen," she said.

On Monday, South African defense minister Nosiviwe Mapisa-Nqakula visited Mandela, and afterward reported the former leader was "doing very, very well."

In January 2011, the elderly statesman spent two nights at a private hospital in Johannesburg for what officials initially described as tests, but turned out to be an acute respiratory infection.

Mandela was diagnosed with tuberculosis in 1988 while at Pollsmoor Prison in Cape Town.

In-depth series from GlobalPost: Mandela's Village: South Africa's road from Qunu