Lungs from Autopsy - Cause of Death: Pulmonary Embolism
You can see the massive thrombus in the main artery of the lung, which led to a sudden death with no preceding symptoms.
Most pulmonary embolisms are caused by blood clots in the deep veins of the legs, but can sometimes be caused by introduction of air into the blood stream, embolization of fat, or amniotic fluid.
The Pathology of Influenza. M. C. Winternitz, Isabel Wason, and Frank McNamara, 1920.