The Klondike gold rush, sparked in August 1896, brought tens of thousands of people from around the globe into the Yukon. Whalers had reached the Mackenzie Delta from the Pacific in the same decade. The demographic wave crested in 1900, the same year as a combined influenza and measles epidemic spread through Alaska into the Yukon, known as the Great Sickness. Oral histories and archival evidence show that it was not the absence of immunity but rather the synergistic effects of multiple pathogens that produced this devastating epidemic and its consequences.