Possible BEFORE modern medicine. Not any longer.
Advances since WW2 will permit any such epidemic to be quashed with a tiny fraction of the deaths that would have occurred with current populations WW1 era technology. Rapid communication, modern quarantine methods, rapid vacine development and delivery, anti-viral drugs, ... all will be used together to put the brakes on it FAST.
However, I still would not be the slightest bit surprised if there were a couple of million deaths, mainly in the underdeveloped countries. These would be due largely to the poor roadways and generally backward nature of education and communications.