Pericarditis is inflammation of the pericardium that commonly occurs between ages 20-50. It is usually caused by viral infections but can also be due to other infections, immune reactions, medications, or unknown causes. Diagnosis requires at least two of four criteria - chest pain, pericardial friction rub, ECG changes, or pericardial effusion on echo. Treatment focuses on relieving pain, treating the underlying cause, and preventing complications like cardiac tamponade through NSAIDs, colchicine, steroids, or pericardiocentesis.