Let's try again. You said the Spanish Flu virus doesn't exist anymore - it had gone extinct because mutations are harmful.
My point (supported by the science if you bother to check) is that the Spanish Flu was one strain of the Influenza A/H1N1 virus group which still exists today. Not only that, but the 2009 'Swine Flu,' according to my sources, is a descendant of the Spanish Flu strain. Therefore, the Spanish Flu strain survived long enough to reproduce and mutate, i.e. evolve, into new strains over the next 90 years. Coming to your quote, the original Spanish Flu H1N1 strain has been extracted from human remains (those that died from the disease in 1918) and its genome sequenced ... which has helped virologists trace the 1918 strain's evolution into modern H1N1 strains. Get it?
I know most of this paragraph will go over your head but pay close attention to the part in red bold.
"Although the pandemic influenza viruses of 1957, 1968, and 2009 are all descended, via different pathways, from the 1918 virus, only the 2009 pandemic virus expresses an antigenically similar hemagglutinin (HA) (11). All influenza A viruses (IAV), including the 1918 virus, possess a segmented single-stranded RNA genome and can evolve by the accumulation of selected mutations (“antigenic drift”) or through the exchange of gene segments by reassortment with other influenza viruses (“antigenic shift”). Sequencing the 1918 virus provided the basis for the subsequent understanding that the key 2009 virus HA gene, after having apparently been transmitted from humans to pigs in or about 1918, had been maintained in pigs over the past 90 years or so as a separate lineage from the 1918 human pandemic H1N1 virus (11), a lineage that has long been recognized as the “classical” swine H1N1 influenza virus. When the 2009 pandemic virus emerged in humans with a swine H1 HA gene descended from, and still closely related antigenically to, the 1918 pandemic virus, extensive cross-protection between the 2009 and 1918 pandemic viruses was demonstrated in experimental animals (12–16). " (Another source)
Thus, what you initially said was, at best, misleading. The Spanish Flu virus didn't die out. It changed.