Fowler, through whom we learn the story of The Quiet American, is an older, cynical man. His description of innocence sums up how this trait is regarded thematically in the novel:
Innocence is like a dumb leper who has lost his bell, wandering the world, meaning no harm.
uses asimile to liken innocence to a leper: this is, at the very least, an unflattering comparison. A leper suffers from a contagious disease that causes skin bumps, disfigurement, and deformity. Lepers were traditionally isolated from others so as not to pass on the disease and forced to wear bells so that people could hear them coming and get out of the way.
In this case, however, innocence is not only represented as a dangerously diseased individual; it is "dumb." Dumb means both unintelligent and unable to speak. Fowler pictures this person, who can't communicate, wandering the earth with a frightening, contagious disease—yet meaning no harm.
Of course, a leper represents a high potential for harm, whether he "means well" or not. In comparing innocence to good intentions, Fowler shows that good intentions are irrelevant.
Alden Pyle, the quiet American, is the figure of innocence/leprosy/disease in the novel. His blinded, unthinking work to uphold "democracy," which leads him to become a CIA agent, is shown to be as dangerous as leprosy to the Vietnamese people. Fowler turns against Pyle as he realizes his terrorist attacks, meant to advance abstract ideals of freedom and democracy but in reality killing and disabling innocent people. In his dangerous innocence, Pyle does not seem to realize that his support of abstract principles actually robs his victims of those principles by taking away their lives.