Half a billion people were infected.
Some parts of Asia had 30 times higher death rates than some parts of Europe, and generally, Africa and Asia had higher rates, while Europe and North America had lower ones.
A third wave in spring of 1919 was more lethal than the first but less so than the second.
It infected an estimated 500 million people and killed 50 to 100 million of them in three waves.
Sometimes a virus contains both avian-adapted genes and human-adapted genes.
But, during the first, second, and third waves of the pandemic, the mortality shifted.