Diphtheria infections affected more than 200,000 people in 1921 and caused approximately 15,520 deaths that year, according to the CDC.
Diphtheria initially causes infected people to develop a sore throat, weakness, and swollen glands. But then a grey-colored mucus begins to cover the back or their throat. If the bacteria spend enough time in the bloodstream, the infection can produce toxins that cause permanent nerve damage and heart failure.
The disease spreads when an infected person sneezes, coughs, or leaves saliva droplets on surfaces or objects. For that reason, people living in close quarters, particularly in dense cities, were highly susceptible to the disease in the early 1900s.
The diphtheria vaccine was introduced in the 1920s, so natural outbreaks haven't been an issue for nearly a century in the US. Less than five people in the entire country caught the disease over the past decade, according to the CDC. However, the disease remains common in some developing countries in which people don't have access to vaccines.
The CDC recommends children receive four doses of the diphtheria vaccine, followed by booster shots every 10 years to ensure full protection.