The flu can be serious. Strains often mutate quickly.
Apparently, the 1918 flu pandemic lasted 2 years . It infected 500 million people across the world, including remote Pacific islands and the Arctic, killing over 50 million people.
My grandmother died as a result of this flu epidemic in late October 1918 ... just a couple of weeks before WWI ended on November 11 in Europe. She left behind a husband and two young children ... the oldest at five was my Dad.