Gustavus van Leeuwenhoek

 was a Dutch assassin.

He used an air gun to contaminate his targets with the plague samples of his scientist brother, Anton van Leeuwenhoek. The victim would die in about a week unaware of the cause. This mode of assassination would be kept in secret even from the client.

Prince Rupert hired Gustavus van Leeuwenhoek to assassinate Oliver Cromwell during the latter's speech in the Commons. The Second Doctor and Ben Jackson managed to thwart the plot without drawing anyone's attention by confronting van Leeuwenhoek directly. The Doctor was able to guess how van Leeuwenhoek was killing his targets by connecting his name with his brother. (PROSE: The Roundheads)

Behind the scenes
The English pronunciation of the last name is /ˈLeɪvənhʊk/.