Diphtheria is a bacterial infection that can cause a thick gray coating on the throat and tonsils, swollen neck lymph nodes, difficulty swallowing, sore throat, cough, wheezing, and fever. In some cases, it can also cause ulcers covered by a gray thick membrane, redness, swelling, and pain. Diphtheria can be transmitted through airborne droplets, contaminated personal or household items, or coming in contact with skin lesions of an infected person. It is important to note that diphtheria can lead to severe complications such as inflammation of the heart muscle, damage to the nerves, and respiratory failure.