For the critically ill, this is a disease of such severe inflammation and blood clotting that it attacks multiple organs and causes life-threatening problems that cascade around the body.
"The volume of this is, of course, unprecedented in the era of modern medicine," says Ron Daniels, an intensive care consultant at hospitals in Birmingham.
"But it's also the type of illness which is so distinct, and the way it's really different from almost every other patient that we've ever seen before."
"As a doctor it seems at times quite horrific, we have had so many very, very sick patients who are having these profound changes in their body en masse," says Beverley Hunt, a professor of thrombosis - the clotting of the blood - who works in intensive care at a leading London hospital.
"We're all struggling to understand it better and it's absolutely key that we get more research done so we can understand what is going on."
