>>113521379In current estimated lethality and contagiousness it's probably most comparable to the so-called "Spanish Flu" of 1918 (Spain wasn't actually especially hard-hit, just the only country that wasn't censoring the case numbers and impact at the time), one of the most deadly pandemics in history, which killed 50 Million or more with a 2% case fatality rate.
A weak and heavily censored first wave which failed to take heavy quarantine precautions did not kill that many at the beginning of the year, and killed mainly the elderly. But later in the Fall, the virus mutated to a new strain that killed primariy 20-40 years olds and generally hit young people with strong immune systems higher due to a cytokine storm triggering effect.
So far, coronavirus has followed its pattern pretty closely, but no idea about mutations. This kind of mutation would be a worst case scenario. Medical care was much worse but spreading globally much harder in 1918.
https://en.wikipedia.org/wiki/Spanish_flu 
