Why did the Iditarod race start?

This site will explain the history of Dogsleding and the Iditarod in Alaska:

* Iditarod History

Iditarod History. The Beginning. The Iditarod Trail Sled Dog Race first ran to Nome in 1973, after two short races on part of the Iditarod Trail in 1967 and ...

** http://www.iditarod.com/learn/history.html

Because an epidemic of diphtheria came to nome and they didn't have enough anti-toxin to give to everyone. so people outside of nome formed a relay to get the anti-toxin to nome.