According to the World Health Organization, today, the flu affects three to five million people per year and can cause 250 to 500 thousand deaths annually. Though still deadly, these numbers suggest a vast improvement compared to the early 1900s. In 1918, a new strain of influenza trickled in from Europe and ravaged the nation, killing 650,000 nationally and nearly 100 million internationally. During the deadly sweep, makeshift hospitals — or more accurately, camps of tents with medical supplies — were set up to treat the many afflicted. In this picture, ghostly-looking masked doctors and nurses bustle around patients as they lie lifeless on cots.