Infections of the pericardium and the pericardial space with fevers, chest pain, diffuse ST and PR changes on EKG, signs of cardiac tamponade; echocardiogram will aid in making the diagnosis.
Depends of the organism: if infectious it is usually either hematogenous or direct extension. There is an idiopathic recurrant form of the disease.
While any organism can cause pericarditis, here are some common and/or interesting causes:
Viral: enterovirus (Coxsackie B).
Parasite: E. histolytica (liver abscess ruptures into the pericardium).
Chronic disease depends on the clinical suspicion.
Relevant links to my Medscape blog