 The 1918 flu epidemic began with a wave of mild cases in the spring , followed by more deadly waves in the autumn , eventually killing hundreds of thousands in the United States and 50- 100 million worldwide . The great majority of deaths in the 1918 flu pandemic were the result of secondary bacterial pneumonia . The influenza virus damaged the lining of the bronchial tubes and lungs of victims , allowing common bacteria from the nose and throat to infect their lungs . Subsequent pandemics have had many fewer fatalities due to the development of antibiotic medicines which can treat pneumonia . 
