*sigh* while it is certainly possible that someone affiliated with an anti-virus company could do something so unethical. Many of the main people in the research and development portions of the anti-virus companies, used to be programmers/hackers back in the 80's and have real issues with people who write malicious code. (Please note that the original term "hacker" referred to someone who took apart a program and rebuilt it to do something else entirely, NOT someone who broke into other computers.)
Oh and if any of the companies started doing that, do you know how fast the other companies would be jumping on them on it? Not to mention the fact that most virii that are written have the writers "fingerprints" on them so to speak. The authors have a habit of putting calling cards in the code so they can show off what they created.
Most of the virii that are out "in the wild", so to speak, are actually created using virus construction kits, and are created by people who do not actually have a real background in computers. "Script kiddies", people who use others peoples code to try and look like they know how to program. Every now and then a truly nasty virus is discovered, that is new... but these are normally being written in other countries. Very few of the new virii that are actually nasty are coming out of the U.S. anymore.
An important note, most of the anti-virus companies out there today are actually linked to software security companies and are not considered the main profit sources for the companies. (At least the major players in the field.)
Oh another thing, Spam is not a virus... it is annoying but it is not a virus.
A former "bug buster". I was one of the people who used to identify new virii when they first started infecting in the area I was in, find how to remove them and if they were not yet listed on the antivirus sites, notify them of the new virus... how to fix it... and if requested, snail mailed them a copy of the virii and the fix for it. (That is notifying all the major anti virus companies to let them know as a courtesy. Many techs that were the anti virus specialist for their shops did the same thing.)