Malware, including viruses and worms, can distribute themselves as email attachments. They are often capable of harvesting email addresses from address books etc. on the computers that they have infected and sending out messages to these addresses.
Some will simply appear to come from the "
From:" email address of the account on the infected computer. In this case email may be bounced back with a warning that the sender has a virus infection or that the infected email was sent to an invalid email address.
Others will spoof (i.e. forge) the "
From:" email address, using one of those it has harvested at random. As a result any email that is bounced back will go to the email address of an innocent third party who will not have sent out the original infected email! The third party who receives this is often puzzled and may worry that their machine has the virus infection.
If you receive an email warning which contains the name of virus or a piece of malware, you can check the name against McAfee's virus information or F-Secure's Threat Descriptions list and find out how it behaves. If it does not spoof then you were the sender and your machine has an infection; if it always spoofs the "
From:" email address then it is someone else that is infected. In any case you should be always be running up-to-date anti-virus software.