Not every zombie is affected by the contagion in the same way. Your "typical" undead is mostly just a mass of human muscle tissue supported by bones - almost all the organ systems and higher brain functions are inactive. This results in a slow, clumsy, stupid beast of a creature mostly dangerous due to simple endurance and force of numbers. Huntsmen are not your typical undead.
Also known as "runners," due to their ability to do just that, Huntsmen are zombies whose body systems have not fully shut down after death. Whether by a quirk of biology or some strange mutation in the undead pathogen, their sensory, circulatory, and endocrine systems remain in perfect working order. The end result is a quick-moving, eagle-eyed, relentless hunter of prey. Even when survivors know to expect their otherwise-surprising agility, the Huntsmen can easily prove lethal.
From a distance, these zombies can be differentiated from their slower cousins by their constant movement and small numbers. They fidget and shift, scratching at their bodies, all the while constantly snarling quietly to themselves. Unfortunately, if you can see them they almost certainly are about to notice you; once they do, they will never pause in their pursuit. Those who aren't taken by surprise are simply run to exhaustion, then slaughtered. Thankfully, the rarity of Huntsmen means they are never found in the large hordes that Shamblers are - they indeed avoid their brethren, for some unknown reason. If they cluster together at all, it is merely in a "pack" which numbers typically less than five. Then again, that is typically more than enough.