By early 1920, the Spanish flu pandemic had largely subsided. The virus had infected about one-third of the world's population and caused an estimated 50 million deaths globally. The pandemic had a profound impact on public health policies and led to significant advancements in virology and epidemiology.

The legacy of the Spanish flu pandemic includes its influence on public health infrastructure and policies worldwide. It highlighted the need for better disease surveillance and response systems. The pandemic also spurred research into influenza viruses, eventually leading to the development of vaccines and antiviral drugs.

The Treaty of Versailles was signed on June 28, 1919, officially ending World War I. The treaty's signing marked a significant geopolitical shift, but the Spanish flu continued to affect populations worldwide. The pandemic had already started to decline by this time, but its effects were still felt in many regions.

A third wave of the Spanish flu emerged in early 1919, though it was less severe than the second wave. This wave continued to cause significant illness and death, particularly in Australia and other parts of the Southern Hemisphere. The third wave lasted until the middle of the year before subsiding.

The armistice ending World War I was signed on November 11, 1918. The end of the war allowed for better allocation of resources to combat the Spanish flu pandemic. However, the movement of troops returning home continued to spread the virus to new areas, prolonging the pandemic's impact.

October 1918 marked the peak of the Spanish flu pandemic, with the highest number of deaths occurring during this month. In the United States alone, approximately 195,000 Americans died in October. Hospitals were overwhelmed, and public health measures such as quarantines and mask mandates were implemented in an attempt to control the spread.

Despite warnings from health officials, the city of Philadelphia held a Liberty Loan Parade on September 28, 1918, to promote war bonds. The event drew a crowd of 200,000 people and is believed to have significantly contributed to the spread of the Spanish flu in the city, leading to a severe outbreak.

The second wave of the Spanish flu began in September 1918 and was far deadlier than the first. This wave was characterized by a higher mortality rate, with many victims dying within hours or days of developing symptoms. The second wave spread rapidly across the globe, causing widespread panic and overwhelming healthcare systems.

By June 1918, the Spanish flu had spread to Europe, with outbreaks reported in France, Germany, and the United Kingdom. The movement of troops during World War I facilitated the rapid spread of the virus across the continent, affecting both military personnel and civilians.

The first recorded case of what would become known as the Spanish flu was reported at Camp Funston in Fort Riley, Kansas, United States. An Army private reported to the camp hospital with flu-like symptoms, marking the beginning of the pandemic that would eventually spread worldwide.