The way they work
"I didn't send a message to this person, in fact I don't even know them! Why did I get a message from an administrator saying my message was infected with a virus, or undeliverable?"
Answer:
These viruses use a technique called "spoofing". Spoof is a term for falsifying the e-mail headers (pretending to be from someone it's not). What happens is that on an infected machine, the virus looks through the users' address books and mailboxes and randomly selects e-mail addresses. It uses these for both the "to" and the "from" fields so that it appears the message is legit. So when a mail server blocks the attachment, or the message, it bounces it to the sender's address even if that's not really where it came from. So it generally means that the recipient of the blocked attachment message probably corresponded with someone that became infected.
Check here and here
if you want more information
"I didn't send a message to this person, in fact I don't even know them! Why did I get a message from an administrator saying my message was infected with a virus, or undeliverable?"
Answer:
These viruses use a technique called "spoofing". Spoof is a term for falsifying the e-mail headers (pretending to be from someone it's not). What happens is that on an infected machine, the virus looks through the users' address books and mailboxes and randomly selects e-mail addresses. It uses these for both the "to" and the "from" fields so that it appears the message is legit. So when a mail server blocks the attachment, or the message, it bounces it to the sender's address even if that's not really where it came from. So it generally means that the recipient of the blocked attachment message probably corresponded with someone that became infected.
Check here and here
if you want more information