President Obama arrives in South Africa today at an extraordinary moment in this remarkable nation’s history. The country’s first black president, Nelson Mandela, is in critical condition in hospital and South Africans are preparing themselves for life after the man who helped transform their country.South Africa would be celebrating the arrival of the first African-American president were it not for the fragile health of the living symbol of the country’s triumph over whites-only rule. Mandela, who is turning 95 on July 18th, is spending his 21st day in hospital. Originally, he was being treated for a lung infection — he is vulnerable to respiratory problems and contracted tuberculosis after 27 years in prison, where he worked in a limestone quarry — but his condition has deteriorated and he is reported to be on life support.