According to noted computer virus expert Paul Ducklin, in order for a virus to be considered in the wild, "it must be spreading as a result of normal day-to-day operations on and between the computers of unsuspecting users." Although there are an estimated 47,000 computer viruses, fewer than 600 are said to be circulating outside of laboratories and research facilities - hence, in the wild. Experts say these wild viruses pose the most significant threat to computers. Wild viruses typically contain a damaging payload and the ability to wipe out all computer files, sometimes even damaging a computer's BIOS.
The Wild List Organization International has compiled a cumulative list of viruses considered to be in the wild. New viral strains are added as they are reported and can be verified. Recent examples of computer viruses in the wild include Melissa, CIH, and W.32 Navidad (the Kriz virus). The Wild List Organization International maintains an updated list of wild viruses. View it here: http://www.wildlist.org/WildList/.