Diphtheria is a rare bacterial infection that targets the respiratory system.  It kills healthy tissue and causes the body to form a thick gray mucus like coating that makes it difficult for a person to breathe or swallow.  If left untreated it can be fatal in a matter of days. 

Several more children came down with Diphtheria and in a desperate attempt Dr. Welch tried to administer the expired antitoxin.  It had no effect and two more children died.  With the bacteria spreading and the antidote shipment unable to be delivered the only thing that could be done was quarantine the town. 

With the town sealed they now tried to figure out a way to receive the antitoxin.  Unfortunately Alaska was experiencing its worst winter in 20 years so primitive airplanes of the time were out of the question and boats were already ruled out due to the harbor being frozen over, no train tracks ran to the town either.  Complicating matters was the fact that the closest supply of the antidote was in Anchorage, Alaska which was over 800 Kilometers (500 miles) away.  Making things even more difficult was that there was no clear path to Anchorage from Nome so the antitoxin had to be shipped by rail to the town of Nenana which was 1,085 Kilometers (674 miles) away from Nome.  The only clear way was across the Iditarod Trail and a trip like that usually took between 25-30 days to make, the antidote when exposed to such harsh weather conditions would expire in as little as six.  They knew they had to figure out something as Dr. Welch said that there would be a 100% mortality rate without the antidote.