What’s even more remarkable about the 1918 flu, say infectious disease experts, is that it never really went away. After infecting an estimated 500 million people worldwide in 1918 and 1919 (a third of the global population), the H1N1 strain that caused the Spanish flu receded into the background and stuck around as the regular seasonal flu.
They never really go away they just evolve into something highly infectious but benign. The line has a hard time continuing if one of its features is kill the carrier.