Short answers, pulled from the story.
The 1918 Spanish flu pandemic was caused by an H1N1 strain of influenza A and claimed an estimated 50 million lives worldwide. This virus spread with terrifying speed across the globe, carried by soldiers returning from the First World War and the chaotic movement of populations during a time of global conflict.
Influenza viruses evolve through two distinct mechanisms called antigenic drift and antigenic shift, which together create a perpetual cycle of seasonal epidemics and occasional pandemics. This constant evolution is why flu vaccines must be updated annually, as the virus is always one step ahead of human defenses.
The first convincing record of an influenza pandemic dates back to 1510, beginning in East Asia before spreading to North Africa and then Europe. The first influenza vaccine was licensed in 1945 in the United States, and influenza A virus was identified as the agent responsible for human influenza in 1933.
Influenza viruses infect 5 to 15 percent of the global population each year, causing 3 to 5 million cases of severe illness and accounting for 290,000 to 650,000 deaths due to respiratory illness. In temperate regions, the number of influenza cases peaks during the winter season, from October to May in the northern hemisphere.
Influenza can be prevented and treated through a combination of vaccination, antiviral drugs, and non-pharmaceutical interventions, but the effectiveness of these measures depends on the timing and the specific strain of the virus. Annual vaccination is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups.