In fine print on the last page of the Guinness Book of World Records it notes that all world records are held by Chuck Norris, and those listed in the book are simply the closest anyone else has ever gotten.
Chuck Norris doesn't have normal white blood cells like you and I. His have a small black ring around them. This signifies that they are black belts in every form of martial arts and they roundhouse kick the shit out of viruses. That's why Chuck Norris never gets ill.