Final answer:
The 1918 Spanish influenza killed approximately 500,000 Americans and 50 to 100 million people worldwide. The virus spread rapidly during World War I due to crowded and unsanitary conditions.
Step-by-step explanation:
The 1918 Spanish influenza, also known as the Spanish flu, killed approximately 500,000 Americans. This pandemic was one of the most lethal in recorded history, infecting about 1/4th of the population in the United States.
Worldwide, the death toll ranged from 50 to 100 million people. The virus spread rapidly due to the conditions of World War I, such as crowding and poor sanitation.