The 1918 flu pandemic (January 1918 -- December 1920) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus. It infected 500 million people around the world, including people on remote Pacific islands and in the Arctic, and resulted in the deaths of 50 to 100 million (three to five percent of the world's population), making it one of the deadliest natural disasters in human history.

Flu that spread over a large area in 1918? OK. Let's think. My stream of consciousness:
To answer this question, we should know that: The 1918 flu pandemic (January 1918 -- December 1920) was an unusually deadly influenza pandemic, the first of the two pandemics involving H1N1 influenza virus. The answer is H1N1 influenza virus.