The 1918 influenza epidemic killed between 50 million and 100 million people worldwide. This epidemic happened near the end of World War I. More people died from the influenza epidemic than were killed in the war. Which of the following explains why this virus was so deadly worldwide?
A. Infected soldiers returning from the war spread the virus when they coughed
B. Medical personnel often became ill as a result of exposure to airborne virus particles.
C. Food was scarce because of the war, so people were undernourished, and their immune systems were weakened.
D. All of the above.