It wasn't originally designed to detect viruses, (either paid or free version) but instead to detect various types of malware that most antivirus programs miss. Although their advertising indicates viruses are detected, it's a poor antivirus detector. As just an example, I've run a few tests in years past and just ran one of them again.. Do a search for the EICAR test file on Google.. Now disable your onboard antivirus program, then download a couple of the EICAR test files to any folder on your desktop. All decent antivirus programs are designed to detect the EICAR test file (AVG, Avira, Avast, McAfee, Norton, Trend Micro, etc.) Now ask Malwarebytes to scan the folder. It won't identify the EICAR test file as malware. Now enable your antivirus program and run a scan of the folder.. It WILL detect the test file. (Just a note: Another malware detection program named "SuperAntispyware" WILL detect the EICAR test file but doesn't delete it automatically as most antivirus programs do.) I've found it interesting they say the reason is because they don't want to put some of the fake and older viruses in their database...and yet they put other fake malware in their database such as the Spycar fake malware.. Just know that Malwarebytes isn't as good at detecting viruses as a dedicated antivirus program.
I've also tested Malwarebytes on a few older viruses which I keep around here for such tests.. It doesn't find quite a few.
That's not to say it's not an excellent tool for removing malware from your computer. I use it all the time. It's just important to know EXACTLY what each tool is looking for. When cleaning out a computer of unknown types of malware, be sure to run a good antivirus program AND a good antimalware program.. Or maybe a couple of both types, just to be sure.
Hope this helps.