“INFECTION, by the classic definition, means that a bacterium or virus or other small organism has entered our body and reproduced itself many times over, causing damage in the process. In response to the damage, we sneeze, vomit, cough or bleed. Even a futuristic definition of infection, which includes the possibility of “gene therapy gone wrong” causing a kind of “infection by DNA,” still requires the presence of genetic material.”
http://www.strangehorizons.com/2004/20040216/killer.shtml