Every year, influenza – or flu – is responsible for three to five million severe cases and 250,000 to 500,000 deaths worldwide. In other words, the "seasonal" influenza that causes annual outbreaks is far from being a "trivial" infection, and it is by no means harmless. "I didn't realize that flu could knock you out so completely," recalls a former flu sufferer, who was "unable to get out of bed" for six days.