The "From" address in email is comparable to the return address on a piece of snail mail. It can be set to whatever the sender desires, and is not necessarily accurate. Just as you could write "1600 Pennsylvania Ave, Washington, DC" on a letter and drop it in the mail, a spammer can set the "From" address to be whatever they want.
Spam, including virus spam, often forges the "From" header in email so that it appears to be coming from someone who has no relation to the mail itself. Often, viruses will pick a random address in the address book of an infected user, set that as the "From" address of the mail, and then pick another random address to send the message to.
These messages should generally simply be ignored. Replying to the sender is not useful, since it is likely forged. The same applies for bounce messages that claim you sent spam to someone else. A virus likely set your email address as the "From" line, and the recipient's mail server sent mail to you to tell you this, even though this is useless.
If you know the spam or virus is coming from an MIT machine (as indicated in the headers by the originating host, NOT the sender's email address), you can forward the message, without attachments, to email@example.com with a note indicating this. Be sure to include the full headers of the message. Network Security will then look into it and inform the owner of the infected machine, and possibly disable network service to the machine.
For information on how to view and forward your full mail headers, see: Q: Forwarding Full Mail Headers