In terms of mortality, which of these strains of flu was the deadliest?

The influenza pandemic of 1918, also known as the "Spanish Lady", was a global disaster that killed millions of people. Exact numbers vary widely, but 20 to 40 million deaths are attributed to this flu. Some estimates are from 50 to 100 million. It is only known as Spanish flu because Spain, neutral in the hostilities of World War 1, was the only country without press restrictions. Across Europe, people assumed their own outbreaks originated in the only place reporting on the disaster.

The war's mass movements of men in armies and aboard ships likely aided in its rapid diffusion and attack. Although there are different theories as to the point of origin, it was first identified at Camp Funston, Kansas, in March 1918. Outbreaks swept through North America, Europe, Asia, Africa, Brazil and the South Pacific and was said to have infected 500 million people.

An estimated 675,000 Americans died of influenza during the pandemic. More U.S. soldiers died from the 1918 flu than were killed in battle during the war.

It is also reported that as many as 17 million succumbed to this flu in India.

One physician recalls that the influenza patients "died struggling to clear their airways of a blood-tinged froth that sometimes gushed from their nose and mouth."

By the summer of 1919, the flu pandemic came to an end, as those that were infected either died or developed immunity. Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain.