JOHANNESBURG (AP) ― 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.”

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.

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.