In the Black Plague of the early Middle Ages about half of the European population died. Please describe why such an outcome would be unlikely today. What kind of epidemic could, indeed, wipe out half of the population in modern Europe or North America? Why?

In the Black Plague of the early Middle Ages about half of the European population died. Please describe why such an outcome would be unlikely today. What kind of epidemic could, indeed, wipe out half of the population in modern Europe or North America? Why?