Former South African leader Nelson Mandela is in "serious but stable condition" after returning to a hospital with a recurring lung infection, the presidential office said Saturday.Mandela was taken to a Pretoria hospital early Saturday after his condition deteriorated, Jacob Zuma's office said in a statement."Doctors are doing everything possible to make him better and comfortable," the statement said.Mandela, 94, has become increasingly frail over the years and has not appeared in public since South Africa hosted the World Cup in 2010.He suffered from tuberculosis during his imprisonment and has battled respiratory infections over the years. His history of lung problems dates to when he was a political prisoner on Robben Island during apartheid.The former president has been hospitalized several times recently for lung infection and pneumonia.Last year, he spent Christmas holidays at the hospital after undergoing treatment for a lung infection and gallstones, one of his longest hospital stays since his release from prison in 1990.Considered the founding father of South Africa's democracy, Mandela became an international figure when he endured 27 years in prison for fighting apartheid, the country's system of racial segregation.In 1993, he and then-South African President F.W. de Klerk jointly won the Nobel Peace Prize.Mandela was elected the nation's first black president a year later, serving only one term, as he had promised.Despite rare public appearances in recent years, he retains his popularity and is considered a hero of democracy in the nation. Last year, South Africa launched a new batch of banknotes with a picture of a smiling Mandela on the front, a testament to his iconic status.Mandela's impact extends far beyond South African borders. After he left office, he mediated conflicts from Africa to the Middle East.