Up to 30% of patients who are seriously ill with coronavirus are developing dangerous blood clots, according to medical experts.
They say the clots, also known as thrombosis, could be contributing to the number of people dying.
Severe inflammation in the lungs – a natural response of the body to the virus – is behind their formation.
Patients worldwide are being affected by many medical complications of the virus, some of which can be fatal.
Back in March, as coronavirus was spreading across the globe, doctors started seeing far higher rates of clots in patients admitted to hospital than they would normally expect.
And there have been other surprises, including the discovery of hundreds of micro-clots in the lungs of some patients.
The virus has also increased cases of deep vein thrombosis – blood clots usually found in the leg – which can be life-threatening when fragments break off and move up the body into the lungs, blocking blood vessels.