Whipworm from pigs appeared to be a good candidate because they don't naturally infect humans and can't reproduce in them.

WSJ: In a Squeaky-Clean World, a Worm Might Help Fight Disease

During the incubation period of seven to 17 days, you look and feel healthy and can't infect others.

CNN: Smallpox

And if you don't use Microsoft Word 97 (or the prerelease version of Word 2000), breath a sigh of relief: Melissa can't infect your computer.

CNN: How to protect yourself against Melissa

Exploiting the preview pane feature in Microsoft's Outlook e-mail program (which allows users to see a message in a small window before they open it) BubbleBoy can infect your PC even if you don't open the attachment it is hiding in.

CNN: From the Web

But it can still infect your computer, and while it doesn't appear to do more than send out more such messages, its other potential effects aren't completely understood yet.

CNN: How to protect yourself against Melissa