Do you think that some anti-viruses (virus-fighters) team up with some virus-makers? Well, I think it's true, by the following things that have happened to me. When I downloaded an anti-virus, five minutes later three viruses started to appear, even though I had the anti-virus. But when I downloaded another anti-virus, it worked perfectly. I've seen that when you download an anti-virus, viruses would maybe appear. One might be a credit-card hacker, so be careful.
An anti-virus couldn't protect itself if the virus-makers want to hack them. That means that the virus-makers could trick the anti-virus because, when they team up, the anti-virus will trust them. But they could hack it easily with permission from the anti-virus to make edits, so they could make it so that the money will go to the virus-makers.
There is really no exact answer -- just some anti-viruses make viruses and some don't. Here are some anti-viruses I have used that I think you can trust:
1. Microsoft Security Essentials