In severe COVID-19 patients, metabolism produces insufficient amounts of certain energy-rich compounds called ketone bodies. However, these energy carriers are needed by two important cell types in the immune system in order to fight the virus effectively. This finding could explain why some people fall ill so much more severely than others. A study led by the University of Bonn (Germany) at least points in this direction. The results have now been published in the journal Nature. They also give hope for new therapies.