"Fever Dream" by Ray Bradbury has a boy taken over by microbes after being sick. The 1948 publication date is appropriate as well.
The story concerns Charles, a fifteen-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 his 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 he 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 floorboard in the carriage, killing them on contact. It appears that he has, indeed, been taken over by the microbes in his body.
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