At the moment there are no self-propagating viruses (and other malware) for the Mac.
What the Mac does have Adware that can suck the life out of a Mac, but, and this is an important but, the Mac owner MUST install it. Often times they are tricked into installing it, but an install still occurs.
The tricks are having an Ad banner on a download page that says "Download" so the user clicks on it and then does the install because that is what they expected they would be doing with the software they downloaded.
The 2nd trick is a download aggregation site puts its own installer around some freeware or shareware and their installer does a side load of Adware that they got paid to include. The user gets the software they want, and Adware on the side.
The 3rd trick is a developer gets paid to include the Adware along with their program when the user installs it.
Again, in all cases the user installed something.
MalwareBytes run _AFTER_ you install something will catch any known Adware or malware. Between doing any installs, it is not necessary to run anything extra.
NOTE: as far as we have observed in these forums, no anti-virus package has actually found any of the Adware and removed it. There have been lots of false positives that scare the user, there have been quarantined files that break the Mac, there have been network access blockages, there have been kernel panics that interrupt what you are doing, but actually doing something useful, we have not seen it.
The one other useful utility has been EtreCheck, both in helping identify things that slow down a Mac (you can find thousands of EtreCheck reports in these forums), and in identifying Adware and helping remove it.
All the other horror stories you have heard are for Windows, and not Macs.
