Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae that can infect the throat or skin. It releases a toxin that causes tissue damage and can affect organs. Symptoms include sore throat, low fever, and formation of a gray membrane in the throat. Complications can include heart damage, nerve palsies, and airway obstruction. Treatment involves antibiotics, antitoxin antibodies, and supportive care. Prevention is through vaccination.