But from time to time endemic viruses may just become way more lethal, as can be seen with the Spanish Flu in 1918-1920 and the Manchurian Plague in 1910-1911
That's not correct. The influenza pandemic was not a new strain of influenza, it was a strain introduced to a naive population of soldiers taken from remote farm areas worldwide and concentrated into one small region of Europe. Then, the soldiers left the war and spread flu all across the remote areas of the US. The flu pandemics are more about naive populations. Same thing happened when Europeans first came to North America and wiped out indigenous populations with influenza.