Diphtheria is a serious bacterial disease that was once one of the most feared childhood illnesses in the world. It causes a thick, gray coating to form in the throat and nose, making it extremely difficult to breathe or swallow. Before modern medicine, diphtheria killed thousands of children every year.

The disease spreads through coughing and sneezing, similar to a cold, but it's far more dangerous. In severe cases, the throat swells so much that victims can suffocate. The bacteria also produce a powerful toxin that can damage the heart and nerves.

Diphtheria became much less common after scientists developed a vaccine in the 1920s. This vaccine is one of the standard shots children receive today, usually combined with vaccines for tetanus and whooping cough. The dramatic decline of diphtheria shows how vaccination transformed public health: a disease that once filled children's hospitals became rare enough that most doctors today will never see a case.

In 1925, mushers raced sled dogs through an Alaskan blizzard to deliver diphtheria medicine to the remote town of Nome, where an outbreak threatened children. This famous journey, which saved many lives, inspired the Iditarod sled dog race that still runs today. The lead dog on the final leg of that journey was named Balto, and a statue of him stands in New York's Central Park honoring that heroic mission.