South Africa's presidency said that doctors were acting with extreme caution because of the advanced age of the anti-apartheid leader, who has become increasingly frail in recent years.
If you don't know about Mandela so well here is a brief about his life & How he became a Living Legend in Africa! Read below...
The Nobel Peace Prize laureate was admitted just before midnight to a hospital in Pretoria, the South African capital. He has been particularly vulnerable to respiratory problems since contracting 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, who became South Africa's first black president in 1994, is a revered figure in his homeland, which has named buildings and other places after him and uses his image on national bank notes.
"I'm so sorry. I'm sad," Obed Mokwana, a Johannesburg resident, said after hearing that Mandela was back in the hospital. "I just try to pray all the time. He must come very strong again."
In December, Mandela spent three weeks in a hospital in Pretoria, where he was treated for a lung infection and had a procedure to remove gallstones.
Earlier this month, he was hospitalized overnight for what authorities said was a successful scheduled medical test.
Presidential spokesman Mac Maharaj, referring to Mandela by his clan name "Madiba," said the latest stay 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 hospitalized so that he can receive the best treatment."
He 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.
In February 2012, Mandela spent a night in a hospital for minor diagnostic surgery to determine the cause of an abdominal complaint. In January 2011, he was admitted to a Johannesburg hospital for what officials initially described as tests but turned out to be an acute respiratory infection. He was discharged days later.
He also had surgery for an enlarged prostate gland in 1985.
The apartheid government released Mandela in 1990. Four years later, he became the nation's first democratically elected president under the banner of the African National Congress, helping to negotiate a relatively peaceful end to apartheid despite fears of much greater bloodshed. He served one five-year term as president before retiring.
Perceived successes during Mandela's tenure include the introduction of a constitution with robust protections for individual rights and the Truth and Reconciliation Commission, a panel that heard testimony about apartheid-era violations of human rights as a kind of national therapy session.
Mandela last made a public appearance on a major stage when South Africa hosted the 2010 World Cup soccer tournament.
Until his latest string of health problems, Mandela had spent more time in the rural village of Qunu in Eastern Cape province, where he grew up. He was visited there in August by then-U.S. Secretary of State Hillary Rodham Clinton.
Doctors said in December that he should remain at his home in Johannesburg to be close to medical facilities that can provide the care he needs.