Military doctors are treating South Africa's former President Nelson Mandela for a recurring lung infection, an ailment the 94-year-old anti-apartheid leader remains susceptible to because of his age and his 27 years in prison.

Government officials acknowledged for the first time Tuesday that the illness forced soldiers to admit Mandela to a military hospital on Saturday, though they said the politician was responding to treatment.

Mandela fought off a similar infection in 2011 and once contracted tuberculosis while imprisoned. Medical experts say respiratory illnesses like pneumonia striking a man his age are a serious matter that require care and monitoring.

"They call pneumonia 'the old man's friend' because it is the thing that ultimately carries many people off," said Dr. Peter Openshaw, the director of the Center for Respiratory Infection at Imperial College's National Heart and Lung Institute in London. "What I guess they'll be doing is trying to find out exactly which type of infection it is and then to give it the most appropriate treatment. With modern antibiotics and investigation, then there's no reason a chest infection by itself should be untreatable."

Ongoing hospitalization worries South Africa

The announcement ended speculation about what was troubling the ailing Mandela. His ongoing hospitalization has caused growing concern in South Africa, a nation of 50 million people that largely reveres Mandela for being the nation's first democratically elected president who sought to bring the country together after centuries of racial division.

The tests Mandela underwent at 1 Military Hospital near South Africa's capital, Pretoria, detected the lung infection, presidential spokesman Mac Maharaj said in a statement.

"Madiba is receiving appropriate treatment and he is responding to the treatment," Maharaj said, referring to Mandela by his clan name as many do in South Africa in a sign of affection.

In January 2011, Mandela was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an acute respiratory infection. The chaos that followed Mandela's stay at that public hospital, with journalists and the curious surrounding it and entering wards, saw the South African military take charge of his care and the government control the information about his health. In recent days many in the press and public have complained about the lack of concrete details that the government has released about Mandela's condition.

Mandela has a history with lung problems. He fell ill with tuberculosis in 1988 toward the tail-end of his prison years, after he had been moved from the notorious Robben Island and to another jail to ease the apartheid government's efforts to negotiate with him about a possible release. At first, doctors were uncertain why Mandela had a persistent cough that ultimately caused him to collapse during a meeting with his lawyer. After being taken to a Cape Town hospital, a doctor told him he had water in his lungs.

Mandela initially refused to believe the doctor, he wrote in his autobiography Long Walk to Freedom.

"With a hint of annoyance, (the doctor) said, 'Mandela, take a look at your chest,"' Mandela recounted. "He pointed out that one side of my chest was actually larger than the other."

Surgeons immediately cut into Mandela's chest and removed two litres of liquid from his lungs, which tested positive for tuberculosis. Doctors at the time suggested Mandela contracted the disease from his damp prison cell.

About 1.4 million people worldwide die each year from tuberculosis, a bacterial infection which can stay dormant for years. It also can cause permanent lung damage, though in his autobiography Mandela says doctors caught it in time. However, tuberculosis can return to trouble those previously infected, properly treated or not, and previous damage could have been missed, Openshaw said.

Treating 'special patient' can be tricky

Openshaw, who has not seen Mandela's medical records and spoke generally about treating patients, said pneumonia is the most likely respiratory illness to affect an elderly person, though others can strike as well. Doctors typically use antibiotics to treat such infections, though there needed to be care made in deciding how much of a dose to give an older patient.

And there's the challenge of treating a patient that a nation and many around the world remain anxiously worried about.

"It's particularly difficult if it's in a special patient, where you really have to be very careful to try not to overreact, but just to treat them as if they were any other patient," Openshaw said.

But the doctor later acknowledged the obvious: "It's very hard to the balance right (for) a special, special patient."

Mandela was a leader in the struggle against racist white rule in South Africa and once he emerged from 27 years in prison in 1990, he won worldwide acclaim for urging reconciliation. He won South Africa's first truly democratic elections in 1994, serving one five-year term. The Nobel laureate later retired from public life to live in his remote village of Qunu, in the Eastern Cape, and last made a public appearance when his country hosted the 2010 World Cup soccer tournament.

Mandela disengaged himself from the country's politics over the last decade but continued campaigning against AIDS. He has grown increasing frail in recent years.