The most common cause of pericardial effusion is inflammation of the pericardium. The most common cause of pericardial effusion is acute nonspecific pericarditis, followed by more common clinical causes including viral pericarditis, tuberculous pericarditis, tumor pericarditis, uremic pericarditis, septic pericarditis, and myocardial infarction pericarditis. Acute nonspecific pericarditis: the etiology is unknown and may be related to viral infection or allergy or autoimmune reaction. There is often a history of upper respiratory tract infection in the weeks prior to the onset of the disease. Tuberculous pericarditis: direct spread from mediastinal or pulmonary or pleural tuberculosis lesions, or spread from lymphatic vessels to the pericardium. Neoplastic pericarditis: Primary tumors of the pericardium are mainly mesothelioma, which is less common. Metastatic tumors are more common and often originate from malignant tumors of the bronchus or breast, and to a lesser extent, lymphomas and leukemias can also invade the pericardium. Purulent pericarditis: It is caused by direct spread of intrathoracic infection, subdiaphragmatic or hepatic abscess penetration, or pericardial penetrating injury infection; rarely, it can also be caused by hematogenous bacterial dissemination. The common causative organisms are staphylococci, gram-negative bacteria, and pneumococci. Myocardial infarction pericarditis: may be an autoimmune process following cardiac injury, with antigens derived from damaged myocardial tissue. Pericardial effusion, if severe, can cause cardiac tamponade and is life-threatening, so it is important to identify the cause and treat it promptly.