How dangerous is it to eat undercooked chicken? i ate half of a chicken breast before i noticed it was not ok. help i'm really worried

Russian roulette: Uncooked or undercooked meat is generally considered contaminated. Chicken is associated with bacteria such as Salmonella (GI flora of chicken)along with other possible bacteria. If the outer part was cooked and the inside not cooked, it would probably be less risky. Usually the process is self-limiting. Best wishes that you will be safe.

Not a worry: If the chicken was fresh, or removed from the freezer, and the outside cooked to whiteness even if the inside was a little pink, then you have nothing to worry about. Bacteria from "bad" chicken are on the surface and rarely inside, so the cooking of the outside will kill them off even if the inside is a bit less well done.