These emails are generated from a variety of viruses circulating around the Internet-they do not actually come from ONS. You receive these when a person's computer gets infected with a virus and you are in that person's email address book. The virus then can pick your email address from the address book and send you the virus but claim that it came from some other address pulled from the infected computer's contacts.
Currently, almost all major viruses fake the email address from which they were sent; this is designed to trick us into viewing the email because we think it came from a trusted source. Also, some of these virus messages may appear to be blank, but they still could contain a hidden virus.