Q: Is security or anti-virus apps necessary?

Unless you're concerned about Windows malware, you shouldn't; instead, avoid downloading software from sources other than the Mac App Store and the developer websites.

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There are no viruses that can attack Mac.OS X. So, anti-virus programs are selling a cure for a disease that doesn't exist. And they are renowned for borking Mac systems.

A Mac can pick up malware but not if you just use common sense. You really have to actively invite some malware in for it to happen. That means downloading and knowingly giving your password for it to download. Mac provides all the protection you need with SIP (System Integrity Protection).I About System Integrity Protection on your Mac - Apple Support

If you do happen to download malware you can use MalwareBytes to get rid of it.

Mac AV is not only unnecessary at this time it is simply not anything near as flexable as Windows AV.

Windows AV has had time and resources to mature because it's a necessity for 90% of the worlds desktop and laptop computer. the developers have had decades to work with Microsoft (Apple Computer does not coordinate efforts with AV developers) and far more developers are versed in Windows.

Mac does not have a anti-virus necessity at this time, and while that truth may change tomorrow what hasn't changes is state of Mac AV has had far less development hours (decades really) far fewer developers and has never been faced with an actual in-the-wild virus so it's effectiveness is entirely theoretical. What has been shown is most if not all Mac AV solutions have caused far more problems from system instability to kernel panics (crashes that cause lock-ups) than any measure of safety they claim. Right now Mac AV is tantamount to a scam if you search these forums to investigate their claimed effectiveness and the actual problems they have caused.

OS X is not Windows, and does not behave as Windows. Windows Virus can not attack OS X because the underlying framework is vastly different. Executable code for a virus in Windows would appear to be gibberish on a OS X and it's payload is not capable of inserting itself into the OS, even if a user manually forced it in it would not be able to do more than be interpreted as a string of random text. For Mac the best polices at this time are to practice safe and responsible browsing and keep your system up-to-date with OS patches and securities patches from Apple: These are the single best thing you can do to protect yourself and keep your system running optimally.