JOHANNESBURG (AFP) - Respiratory infections like that afflicting former tuberculosis sufferer Nelson Mandela are common, but need to be treated seriously, a South African medical expert said Tuesday. The 94-year-old anti-apartheid icon, who for years quarried limestone on Robben Island prison, has been in hospital for four days of tests and treatment. “As one ages, one of the commonest problems of elderly people is chest infection,” said Mark Sonderup, a medical doctor and vice-chair of the South African Medical Association. “It wouldn’t be unusual,” he said, but must always be treated seriously. Lung infections are caused either by bacteria or viruses. The symptoms can be similar, including coughing, fever and difficulty breathing.