WHY most people who get COVID have mild symptoms or none at all while some become severely ill is still a mystery – a mystery that scientists are urgently trying to solve. REBECCA AICHELER, Senior Lecturer in Immunology, Cardiff Metropolitan University Being obese or having existing health problems, such as diabetes or high blood pressure, are known to increase the risk of severe COVID. But this is not the whole story. Some seemingly healthy people can suffer from severe disease, too. Early in 2020, scientists discovered that people with severe COVID had unusual levels of certain immune cells in their…