If a person is not vaccinated and the disease is left untreated, the bacteria may produce powerful poisons causing a plethora of serious side effects. The bacterium Corynebacterium diphtheriae. It lives in the nose, throat and mouth of an infected person, so it's easy to spread by coughing, sneezing, kissing, etc. It can also be spread by e.g. touching open sores of people who have skin diphtheria. Incubation period is 2 to 7 days.
Early symptoms are a slight fever, chills and a sore throat.
The disease mainly develops in the throat where it can form a leathery membrane, leading to suffocation. If left untreated the bacteria will start to produce a powerful poison which may cause such side effects as heart failure, respiratory failure, pneumonia or paralysis. In the case of skin diphtheria, the patient's body is covered with a rash and possibly ulcers and sores. Diphtheria generally occurs more frequently in the colder months and is associated with crowded indoor living conditions. Avoiding these lowers the risk.