Like I do for Windows, I recommend a two-pronged defense that includes a good antivirus program combined with a good anti-malware program, so in addition to one of these I would also consider installing the Mac version of Malwarebytes from /mac to add that extra layer of protection. If you are looking for a no-cost solution, you can check out Avira Free Antivirus from /free and Sophos Home from. At this site you can read reviews and make an informed decision about which antivirus program you might want to use for your computer. Macworld publishes a list of the best anti-virus applications for the Mac at /helplinemacav. But threats to the Mac OS are on the rise, so it is a good idea to keep antivirus and anti-malware software installed on a Mac just to be on the safe side.
This is one of the reasons that antivirus software on a Mac can seem unreliable.