Illustration of a cytokine storm response to infection with the new coronavirus SARS-CoV-2 (red spheres). A macrophage (purple) white blood cell is releasing inflammatory immune proteins (cytokines, white) that are causing cells to become inflamed (red). A cytokine storm is a severe immune reaction that results in greatly elevated levels of cytokines in the body. The high levels of inflammation cause damage and can be fatal, even in those that were otherwise fit and healthy. SARS-CoV-2 emerged in Wuhan, China, in December 2019 and spread throughout the world. The virus causes a mild respiratory illness (covid-19) that can develop into pneumonia and be fatal in some cases.