I believe Malware or Annoyanceware come from many places. Ads on Google and other sites, especially free software, games, coupon and music sites. Avoid Coupon sites, I think the printing services they install on your PC boarder on Malware and track your activity and influence your web searches.
1. Install a Good Antivirus\Antimalware like Bitdefender Free, Avast Free or Sophos Free on MACs. (malware refers to all threats, including viruses)
I personally like the free ones and I use Bitdefender Free. If you want a little extra protection, the pay version of these are excellent also and so is Webroot.
If you want a little extra pay protection download HitManPro 3 for a second opinion malware scanner. It is free for 30 days. I’ve found and eliminated threats with this that no other scanner found.
In a recent PC Mag report Bitdefender and Webroot ranked at the top of pay Antiviruses. I don’t like Norton or McAfee, they are too bloated and intrusive.
2. Run a full scan with your Antivirus once a month. These are usually manual. Most Antiviruses run quick scans on a regular basis, but never a full scan because it can take hours.
3. Once a Quarter or as needed
a. Run Free Adwcleaner
b. Run Malwarebytes Free
c. Run Advanced System Care Free
d. After running them, uninstall all three so they don’t annoy you with upgrade requests etc.
Let me know if you have any questions.