Acute pericarditis. Light micrograph of tissue from a heart in a case of acute pericarditis, inflammation of the pericardium (the sac containing the heart). Pericarditis can be caused by fungal, viral or bacterial infection, an injury, cancer, a heart attack, or an autoimmune disorder. The inflammation has caused an exudate (across top) to form on the pericardium, which is the thin layer (dark pink) below the exudate. Below the pericardium is the thicker layer of the epicardium, and at bottom the dark pink stain of the myocardium (heart muscle).