"The Hunted" refers to those who have been bitten by vampires, but have not yet been turned (see SFM Virus). They are especially susceptible to recurring attacks, since vampires can smell them miles away. The only way to block this scent is through a daily dose of garlic tablets.
Some of The Hunted have inherited a portion of the vampire's powers (Slayers) - the ability to regenerate (invulnerability) along with heightened reflexes and strength. These powers are enhanced by the consumption of blood. The Hunted can also sense the presence of vampires, which resembles a "chill up the spine" that most people have felt, but typically ignore.
The insatiable desire for blood. Much like a drug addict, once they start drinking blood, it's almost impossible to stop. Cold turkey is an ugly and dangerous thing which few survive. Even rare hamburgers can trigger the addiction. Unlike vampires, however, the Hunted do not require blood to survive, and can learn to cope with their blood addiction through programs such as the VA.
While the Hunted have inherited some of the vampire's powers, they are still not vampires, and do not possess any of their other special powers. They are not immortal - they will age and eventually die. And even though the Hunted have accelerated regenerative powers, they can feel pain and be killed by having their head cut off.
Unlike vampires, the Hunted do not possess glowing eyes or fangs. There are typically two red teeth marks on their neck (or somewhere on their body) that never heal completely. Otherwise, it is virtually impossible to tell if someone has been bitten.