It's important to realize that AV software gives extremely self-serving "malware" alerts. In other words, the products tend to do what they can to scare you, rather than assuage your fears and educate you.
So, for instance, if your AV software comes across a bit of Windows malware that you received as a file attachment to an e-mail message, it generally won't tell you that the "malware" that it found can't run on a Macintosh and is entirely benign.
Another problem is that many commercial AV products don't look for adware at all. Of if they do, they report it as being equivalent to malware. However, even though adware can be extremely annoying, it is in no way malicious or harmful to your data.
Malwarebytes (free)
https://www.malwarebytes.com/antimalware/mac/
does a really good job of detecting and eliminating adware. And it's nice to have because it's free. But I don't think that it looks for any actual malware, even extinct malware, at all. The program runs too quickly to do a thorough scan for malware, and it's too small to contain the necessary malware definitions.
If you want a comprehensive anti-virus program (and I don't think that most Mac users need one), and you want a free one, you may want to check out:
avira (free)
http://www.avira.com/en/free-antivirus-mac
It did pretty well in the last AV comparison test that I find believable:
http://www.thesafemac.com/mac-anti-virus-testing-2014/
I haven't tried it myself.
I can't recommend Avast, because it installs adware.
Usually, for those who insist that they need anti-virus software, I recommend:
Intego's VirusBarrier
VirusBarrier wins all the believable anti-virus program comparison tests (there are lots of shill sites on the Web)
It is currently only $40 and I think that it is worth the money to have the best product if you feel that you need one:
http://www.intego.com/antivirus-internet-security-x8