Diphtheria is a serious bacterial infection of the nose and throat caused by Corynebacterium diphtheriae that is usually spread through respiratory droplets. It can lead to difficulty breathing, heart failure, paralysis, and death if not treated. Symptoms include a thick gray coating in the throat, sore throat, fever, and difficulty breathing. Complications arise due to toxins affecting organs like the heart, nerves, and lungs. Diagnosis involves examination and testing of throat samples, while treatment consists of antitoxin, antibiotics like penicillin for 14 days, and sometimes hospitalization. Vaccines can prevent diphtheria.