They're not in contradiction to each other, and the second link doesn't kick off with that sentence. It kicks off with: The "Spanish" influenza pandemic of 191

They're not in contradiction to each other, and the second link doesn't kick off with that sentence. It kicks off with: The "Spanish" influenza pandemic of 1918–1919, which caused ≈50 million deaths worldwide, remains an ominous warning to public health. Many questions about its origins, its unusual epidemiologic features, and the basis of its pathogenicity remain unanswered. Which... matches pretty well with the first link. Sure, the outcome of the disease was severe, but we don't know why! And recent studies imply that it was because of sociological phenomena.