This. This 100%The problem is not so much "viruses" as it is an ID10T error. Users do it to themselves with all the crapware they download including Flash Player (in spite of all the warnings).
...and stop downloading nonsensical software designed to make your Mac "faster" or "better". None of it works.
Even with the Windows boxes I've worked on, it seems the majority of malware/crap that people get on their computer is a side effect of what's between the seat and the keyboard. And regarding the "better" and "faster" claims that all those wares and suites offer for your mac... it's downright entertaining to see what they claim. But yeah, a complete waste of disk space, and typically they bog down your computer as it is.