Shinkaigyo

Viruses vs. A Shark-type Artificial Creature! To protect humanity from the threat of viruses, the genius scientist Kyou Rouhan created K-1, a therapeutic shark-type bodily sea fish that can swim freely through the human body. However, K-1 has a vicious personality, and is a dangerous being who could become an enemy of humanity… But then, by chance, K-1 combines with a kind-hearted boy who cares for his younger sister!? A marine sci-fi fantasy about fighting viruses! (Source: Shueisha, translated)