Kyuketsumadan

"Tuberculosis" depicts the adventures of Kenichi and his uncle inside a boy's body.
After being injected with a drug called ZX that was discovered by Kenichi's uncle, Kenichi and his uncle have shrunk and become very small. When they enter the body of a boy and reach his lung, they find that tubercle bacilli (bacteria) are opening a big hole in the boy's lung. Tubercle bacilli send Mode (a tubercle bacillus), a daughter of Dr. Iyoa (also a tubercle bacillus), to Kenichi and his uncle to learn the human language. The two make friends with Mode, but a war breaks out between the tubercle bacilli and the commandos in the boy's brain, and the two become involved in the war.

(Source: Tezuka Osamu Official)