Darkseekers (also called Hemocytes) are infected humans. The origin of this infection was a supposed cure for cancer.
Following the failed containment of the initial outbreak on Manhattan Island, the infection spread around America.
and gave the alpha male the specimen then it walked out with the rest of the Darkseekers. This allows us to presume that the Darkseekers are more human than we expected.
The major weakness of the Darkseekers was the unique side effect of their viral mutation, intolerance of ultraviolet light. When exposed to minute amounts to UV light their skin is rapidly eaten away and when exposed to full on UV radation of the sun they are killed within moments.