The pericardium is the thin sac that surrounds the heart ("peri" means around, and "cardio" means heart). Pericarditis is an inflammation or irritation of this sac. Normally, the pericardial tissue protects and supports the heart. The tissue has two layers with lubricating fluid in between the layers. When the pericardium is inflamed, the two layers rub against each other and against the heart itself. This friction causes pain. Viral infections are the main cause of pericarditis, but other possible causes include: bacterial or fungal infection, cancer, kidney failure, rheumatoid arthritis, recent heart attack, and traumatic chest injury. The underlying cause often remains unknown.