Only the 1918 Spanish Flu pandemic came close in mortality figures
In relative population, the 'Justinian's Plague' in the 6th century was easily more devastating to Europe. And, this is the usual Eurocentric POV

anyway - there were lots of equally lethal pandemias in Asia quite frequently, and remember something what, in an alternative history, would be called 'The Great Measles' and 'The Great Tuberculosis' killed about 90% of the native South/Mesoamerican population.
But you're correct about the infection path. On a small note, not sanitation or better hygienic habits saved the Europeans later: The
rattus rattus, the carrier of the fleas, and the usual rat population at the times of plagues, was pushed back by the nowaddays
rattus norvegicus, which isn't the natural carrier of the vector.
On topic:
The really awesome thing here is that c3c allows to choose who's affected from plague. Why? Because the Kings needed to be immune in MA

...
So, if it bothers you, just remove things like Artillery or Submarines from the list.