I’m no expert on viruses, but it seems to me that getting your code to be executed somewhere is the main challenge. For viruses that spread via email, this usually involves exploiting a vulnerability in the email client. Once you’ve achieved this, doing something malicious is trivial, so I recall many early viruses that simply sent emails of themselves to the entire address book.
When the providers of email client software got better at removing the vulnerabilities, it became harder for email viruses to be automatically executed, so a new method was needed. This is how the most prevalent and easily exploited vulnerability was discovered. It just so happens that this bug is the hardest one of all to patch. That’s because it’s in the human brain.
No need to spend hours finding a possible buffer overflow in outlook. No, just offer the recipient of your email the idea that forwarding on your email about danger from “kill someone to get in the gang” week or that sharing your picture status so that “the angel looking over you shoulder” will do something for you, and the suckers will forward/share for you more than you would believe.
It’s the hope that’s important. Big part of belief, hope. Give people jam today and they’ll just sit and eat it. Jam tomorrow, now—that’ll keep them going forever.
Terry Pratchett in Hogfather
So just remember, the next time you’re tempted to share that status, or forward that email. If you do, YOU are the replicator.