Presidential spokesperson Mac Maharaj said he had confirmed during an interview with the BBC that Mandela was conscious when taken to an undisclosed hospital just before midnight on Wednesday.
"In the interview they asked whether when he was taken to hospital he was conscious … I confirmed that."
Maharaj said that given Mandela's age and medical history, the doctors' "acted with the greatest act of caution and responsibility" and made a decision to send him to hospital.
"He is receiving treatment to deal with the infection and to keep him comfortable." Maharaj said infections spread rapidly. Asked if this was the case with Mandela, he answered: "They always do."
Meanwhile, journalists began setting up cameras outside 1 Military Hospital in Pretoria anticipating that Mandela might have been admitted there. Soldiers manned the gate and turned away journalists' cars trying to enter the hospital.
Reporters were at the hospital to cover Deputy Defence Minister Thabang Makwetla's visit to soldiers wounded during a battle with rebels in the Central African Republic last weekend.
Earlier this month, Mandela was admitted to a Pretoria hospital for a scheduled check-up. He was discharged the following day.
In December, Mandela underwent an operation to remove gallstones and to treat the recurring lung infection. He was discharged after an 18-day stay and placed under home-based high care at his Houghton, Johannesburg, residence.
In January, the presidency said Mandela had made a full recovery from the surgery and continued to improve.
In February 2012, he was admitted to hospital with a stomach ailment. At the time, the presidency said Mandela underwent a diagnostic procedure to investigate the cause of a long-standing abdominal complaint.
In January 2011, Mandela was taken to Milpark Hospital for routine tests relating to respiratory problems. Mandela's last major public appearance was in July 2010, at the final of the Fifa World Cup at Soccer City in Johannesburg.
Since then he has spent his time between Johannesburg and his ancestral village of Qunu in the Eastern Cape.
South Africa's first black president has a long history of lung problems, dating back to the time when he was a political prisoner on Robben Island during apartheid. While in jail he contracted tuberculosis. – Sapa