Darkseekers are the main antagonists of the novel, I Am Legend, and 2007 film of the same name. They are the humans susceptible to the Krippin Virus who were infected, but did not die from it.
While those infected with KV who survive it develop primal behavior upon succumbing to the virus and becoming a Darkseeker, they do retain enough of their higher intelligence to build and copy complex traps they have previously seen. Darkseekers' adrenal glands are always open, giving the Darkseekers increased strength and mobility.
However, it also speeds up their metabolism, resulting in cannibalism when food is scarce. The increased metabolism also speeds up Darkseekers' heart rate, breathing speed and body temperature, and makes them constantly hyperventilate.
Other symptoms of KV include dilated pupils, inflamed gums (in infected dogs at least), hair loss, albinism, and a primal state of rage.
Another major symptom is a severe, vampire-like intolerance of UV radiation, causing sunlight and other sources of ultraviolet to burn Darkseekers.
Because of this intolerance of UV, Darkseekers are forced into a nocturnal life cycle; lurking in dark, derelict buildings during the day, and only leaving at night.
The intolerance of UV is also so extreme, direct exposure to sunlight for more than a matter of seconds can be fatal to a Darkseeker.
Darkseekers are also attracted to and crave blood, much like the traditional vampire, and see the uninfected as vicious monsters that must be destroyed.