A zombie refers to most humans that have succumbed to the t-Virus (either through contaminated water supply or infected bites and scratches from a virus carrier). The researchers at the Umbrella Corporation called them "zombies" in reference to a Haitian Vodou practice where witch-doctors are said to reanimate the bodies of the recently deceased for use as slave labor.
During his trials with the t-Virus, Dr. James Marcus began to consider the impact that human testing would have on his research, eventually forcing people into being guinea pigs for his secret bio-weapons projects. The result of these trials was the very first of the zombies. It would appear that information regarding Marcus' deeds only surfaced during the 1998 investigation into his laboratory, which had been quickly abandoned ten years prior when he was murdered. The investigation revealed no less than twenty people as test subjects.