it's actually not, the strain just changed over time to be less and less harmful. The same influenza that goes around every year is directly linked to its ancestor, the Spanish Flu

> "All human-adapted influenza A viruses of today — both seasonal variations and those that caused more dramatic pandemics — are descendents, direct or indirect, of that founding virus," notes Dr. Taubenberger, Senior Investigator in NIAID’s Laboratory of Infectious Diseases. "Thus we can be said to be living in a pandemic era that began in 1918."