The story concerns Charles, a thirteen year old boy who is suffering from a severe illness. The local doctor diagnoses it as scarlet fever, but Charles protests that his hand has "changed" and is no longer under his control. He claims that he has been infected by microbes that are not only causing illness, but literally taking over his body and forming a new being. The doctor, however, assures Charles's parents that this is all in his imagination - a fever dream brought on by his illness.
Charles continues to lose control of his body - first other hand, then his legs - but the doctor continues to assure him otherwise, and gives him antibiotics to deal with his problems. After Charles tries to choke himself, he is restrained to the bed by his parents. One night, Charles begins to lose control of his body, and feels himself being taken over by the microbes.
The next morning, Charles appears fully recovered. He is pronounced completely healthy by the astonished doctor, whose hand Charles vigorously shakes. After the doctor leaves, however, Charles brushes his foot over a swarm of red ants on the sidewalk, killing them on contact, and appears to do the same to his family's pet canary at the end of the story. It appears that he has, indeed, been taken over by the microbes in his body.
Bradbury explores the theme of childhood anxiety and isolation in this story. The disease itself can be read as a metaphor for puberty; the body is changing beyond Charles' control, he is highly anxious and frightened about it, and after it is over, he is a completely different person, who is much more friendly, outgoing, and self-confident. This idea is reinforced by Charles's age being thirteen, right at the time puberty usually begins in boys.
The story also involves childhood fears of isolation and distrust of adults. The doctor continues to scoff at his illness, and Charles's parents do little to help his situation, and perhaps aggravate it. Fear of not being taken seriously by adults is a common worry of children; children in many ways rely on adults to help them, since they are unable to help themselves.
A literal interpretation of the story leads one to believe that Charles has literally been transformed into a walking disease that can kill other living creatures on contact; however, the way the story is written leaves it open to interpretation. It is possible that the entire story was made up of fever dreams or hallucinations by Charles, or that the illness Charles was suffering triggered latent neuropsychiatric disorder - such as alien hand syndrome, which causes his hand to act independently from the rest of his body. However, the final passages after Charles's recovery strongly suggests that Bradbury did, indeed, intend the literal interpretation.