From Hundred Days: The Campaign That Ended World War I, by Nick Lloyd (Basic Books, 2014), Kindle Loc. 650-667:
As well as the thousands of broken and bloodied men that came from the front, there was a frightening surge in cases of influenza across Western Europe. This was the first wave of the great ‘Spanish flu’ pandemic that wreaked such havoc and caused such fear, with up to fifty million people dying worldwide before the end of 1919. Influenza had always been present, but the number of admissions suddenly surged up during the last summer of the war. In the UK there had been somewhere between 2,000 and 3,000 cases each month during the spring, but over 30,000 were registered in June alone. This epidemic took the form of the so-called ‘three day fever’, which was extremely infectious, and, as the British Medical History noted, would strike suddenly ‘so that barrack rooms which the day before had been full of bustle and life would now be converted wholesale into one great sick room’. Patients would experience a high fever, often up to 103 degrees Fahrenheit, before gradually returning to health within a short time. This strain was particularly virulent in the German Army. In the two months of June and July 1918, over half a million soldiers would contract the disease, most of whom were treated in specialized ‘flu infirmaries’ behind the lines. The illness usually began with chills and general malaise, before a fever took hold for 48–72 hours. This ‘lighter’ type of flu was usually not fatal – patients would generally recover within eight or ten days – and had died down by the late summer, and should not be confused with the much more lethal and dangerous strain that emerged over the winter of 1918 and into the following year.
This second strain of influenza was the killer. As the year progressed, Allied and German doctors began to notice new, more terrifying symptoms in their influenza cases. They would soon become familiar with a list of complaints that included bodily weakness and a throbbing headache, chest pains and a hacking cough. Usually blood-stained froth would be brought up and the patient would then show the usually fatal signs of cyanosis – the blue discoloration of the face that meant death was only hours away.