A Dash for a Throne — Arthur W. Marchmont

The young Count von Rudloff has a problem. A big problem. He insulted the Prince of Prussia so severely that he will not be permitted to live. In order to escape the humiliation of a trial and official death sentence and to prevent the world from knowing that the Prince was ever so insulted, he is permitted to kill himself within a week, and to make it look natural or like an accident. He travels to Berlin to see a prominent doctor, an old friend of his mother's, who readily promises to help out by providing the Count with an injection of a deadly virus.