FILE – In this Feb. 26, 2015 photo taken through the eyepiece of a microscope, human cells infected with the flu virus glow green under light from a fluorescence microscope at a laboratory in Seattle. The U.S. government estimates that 80,000 Americans died of flu and flu complications in the winter of 2017-2018 – the highest flu-related death toll in at least four decades. (AP Photo/Ted S. Warren)