A cytokine storm is an over-reaction of the immune system. Cytokines are chemical messengers which control the immune system. And in a cytokine storm they go nuts and you get this overreaction of the immune system. In SARS and SARS-CoV-2, it’s in the lungs themselves and it happens in influenza as well. It’s a serious problem and is the cause of death. It can affect other parts of the body, such as the arteries and maybe even is the cause of blood clotting in COVID-19.
Dr Norman Swan, Physician and Journalist