It only takes ONE stab in the heart to kill you instantly.

Lungs? Not instant, but DEAD.

Any other organs? Not instant but DEAD.

Drop you to your knees immediately either way.

If you are not expecting a chick you were just FUCKING to stab you, and instead think she is filming a porno of you in the shower, you would tend to totally let her stab you two or three times at least before you would really realize that HOLY FUCK SHE FUCKING STABBED ME!

By then it's already too late.

That's how people get killed by stabbing. Often they don't even realize it.