There is frequent fantasy violence/combat against viruses, bacteria, allergens and parasites that are attempting to enter and infect the body. While there is bloody injury detail, there is no excessive gore or brutality.
Full of bloodiness from the beginning of the first episode. Example: viruses being attacked by knife-wielding "white blood cells" and spurting blood all over.
Violence is frequent and bloody in nature, but this is only against grotesque-looking bacteria (though, said bacteria have faces and talk like people). It doesn't dwell on the violence.
There is some over the top blood when the Germs, Bacteria, Allergens, and Viruses are killed. However, the show has a very lighthearted tone. When a dead monster is seen, they have a cartoony X for their eyes. In the show, you rarely ever see any people (Cells) getting killed.