A cause of death for Buffy the Vampire Slayer alum Nicholas Brendon has been revealed. The actor died of atherosclerotic and hypertensive cardiovascular disease, with acute pneumonia and previous myocardial infarction (heart attack) as contributing factors, according to an autopsy report obtained by Us Weekly. Arteriosclerosis occurs when the arteries become thick and stiff while acute pneumonia is an intense, sudden-onset infection that causes inflammation in the lungs.