From Wikipedia (bolding in original):
The danger triangle of the face consists of the area from the corners of the mouth to the bridge of the nose, including the nose and maxilla. Due to the special nature of the blood supply to the human nose and surrounding area, it is possible (although very rare) for retrograde infections from the nasal area to spread to the brain.
Unlike other veins of the circulatory system, veins of the head do not contain one way valves, therefore blood can flow away from the skull and exit the face inferiorly, or flow back into the skull to be drained via the internal jugular vein. This is of concern as any infection within the “danger triangle,” if entered the blood stream, can pass into the brain, thus spreading infection.
Now that’s a fucking great design. Clearly, ID really should stand for incompetent, not intelligent, design.