According to Babylon 5 canon, the original Drakh plan had been to use the Death Cloud on Earth, and then use the plague on Minbar. But when the Death Cloud was destroyed, the plague was the only significant weapon they had left.
The trivia item below may give away important plot points.
Captain Anderson's ship, Charon, is foreshadowing of his eventual fate. In Greek Mythology, Charon is the ferryman of Hades who carries souls of the newly deceased across the rivers Styx and Acheron that divided the world of the living from the world of the dead.