The flu pandemic in 1918/1919 (it came back a second year) infected about 1/3 of the entire world, i.e., it was highly infectious with a huge number of people infected. But it wasn’t a death sentence as the death rate was only 2.5%. Yet millions died because so many were infected. The death rate for the current pandemic is a little over 2%, much greater than the standard flu death rate of just .1%, but, so far, it has infected far fewer people than the 1918/19 pandemic.
A nasty aspect of the Spanish Flu is that it could kill very quickly. Often those people who were infected died that same day or shortly afterward. A respiratory killer — you suffocated from lack of oxygen.