iMac--Do I need virus and/or malware protection?

NO! There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior and make Mac OS appear buggy.

I have been using Macs and Mac OS for over 20 years, in that time I have had zero issues. Please read Apple's stance in:

https://www.apple.com/macos/security/ and also https://support.apple.com/guide/mac-help/protect-your-mac-from-malware-mh40596/mac

There is one app, Malwarebytes. that is good to have if you ever experience any adware popups or what you might think is malware. The free version is all you need and run it if you think you might have something to get rid of.