"The pandemic (a geographically widespread or global infection in people) appeared in all but the remotest places on Earth, causing symptomatic disease in at least one-third of the world’s population. Whereas most sick persons had self-limiting influenza, indistinguishable from influenza as seen today, an unexpectedly high number went on to die of pneumonia. U.S. case fatality ratios were approximately 0.5 to 1%, but case fatality was much higher in many developing countries and in many crowded environments, from urban slums to Western military training camps. The global mortality will never be known with certainty, but estimates range from 50 million to as high as 100 million in the first pandemic year."