That's why Norton, Mcafee, Webroot, etc,etc,etc. sell Anti-Virus utilities for Mac.
And I am sure they are all HUGH sellers, too.
Over ten years of running multiple Macs, up to fifty in one network alone, NONE has had any third-party anti-virus installed. . . and none has ever been compromised! So, that's a very good question. Most people who run Mac AV software—about 2% of Mac OSX users—do it to scrub WINDOWS malware from incoming files before sending them on to their Windows impaired friends. The statistics of even those companies show that 98.7% of the malware intercepted by their Mac AV apps was Windows Malware. . . and the balance was Mac OSX Trojans that could ONLY be found by TURNING OFF the built in protection in OSX so the third-party anti-virus COULD intercept what was already being blocked by the OS. . . and ALL of them disable the Apple supplied, very effective, anti-Trojan system so they can intercept the known Mac malware.
Note that NONE, including Apple's, will intercept a new Trojan in a new family.