The influenza spread rapidly and, like most influenza, seemed to destroy its supply of hosts as it spread. The majority of hosts recovered, but somewhere between 10% and 20% of all cases in the Western world led to pneumonia. And in remote villages that had not been exposed to earlier viruses, the virus proved even more virulent. Combined, it is estimated that hundreds of millions of people developed severe cases of influenza at a time when the population of the Earth was roughly a third what it is today.

All cases had more or less the same symptoms: inflammation of membranes in the respiratory tract, headaches, chills, exhaustion, cough, a high body temperature, and congestion. Some patients had stomach issues as well. But the violent cases came with “pain, terrific pain,” that “could come almost anywhere” (232). One nurse recalled her heart aching and beating rapidly. A scientist noted numbness in his hands and feet. A German soldier recalled severe intestinal pain. Some reported severe earaches. Some loss of smell. Some liver damage. Physicians noted depression and hysteria as common symptoms, and they noted that all patients showed more than one symptom at a time. The variance in the symptoms confused scientists and doctors, as they had never seen a single disease with such diversity of symptoms.