Apple computers are not entirely immune from viruses and malware.
Actually it is, if you accept the definition of the word "immune" to describe a high degree of resistance to viruses. In that context macOS is in fact immune from "computer viruses", a statement supported by the fact that there has never been one that has affected macOS. Not a single one in the history of macOS, which includes Apple's previous name OS X. There has never been a macOS virus, and given Apple's recent security elements incorporated in macOS, I am confident that there will never be a macOS virus.
The reason you had to temper your description with the adverb "entirely" is that while macOS is in fact immune from viruses, it's not invulnerable. The subtle difference is that every now and then, on a slow news day (which today isn't), you will run across a breathless headline in some tech blog proclaiming "First Mac Virus Discovered!!!" with a subheading "Apple refuses to comment" or whatever. Then, buried somewhere in the ninth or tenth paragraph anyone still paying attention will read that the subject virus was directly injected into a compromised Mac through its Thunderbolt port in the same way you'd inject a frightened lab rat with a known pathogen. Physical access to a deliberately compromised Mac makes a variety of nasty exploits possible, and physical inaccessibility is only the first layer of what needs to be an overall, multifaceted security strategy. Delegating that security to a non-Apple "anti-virus" product is the height of irresponsibility.
Apple is the sole source for Macs and the operating systems they run. As the most famously secretive organization in the world, no one knows more about them than they do. Anyone that claims otherwise ought to be regarded with extreme suspicion, if not outright derision. They are all opportunists with one and only one goal: to take your money, whether their products are "free" or not.
It's been twenty years since macOS's inception with the very first OS X Public Beta. Even Microsoft Windows, once the gold standard playground for viruses, has effectively rendered them obsolete with improvements in their PC target systems. Meanwhile "anti-virus" peddlers continue to beat their heads against a wall in a desperate attempt to remain relevant in a post-"computer virus" era. After decades of continual improvements, do you really think anything's going to change?
Words mean things. If you want to discuss malware, that's a different subject. Malware is a broad category of junk, and "junk" is anything that does not convey or support your ability to use your Mac for whatever you need it to do—whether that's productivity or communication or entertainment or information or organization or other beneficial purposes that only you can know.
There is plenty of Mac malware and there always has been. The overwhelming majority of that malware is packaged and distributed in the form of "anti-virus" products. They are junk.
Without having (what I would call) a defined anti-virus, how does one identify and - more importantly - eliminate any of the damaging intruders?
You don't install them. You are the owner and operator of your Mac and only you can decide what you want to put on it.
Mac malware is not the product of spontaneous generation. It doesn't arise out of nowhere. It doesn't insert itself as a mere consequence of receiving an email or message. The Mac malware that has become so popular in recent years requires and depends upon your active participation and consent. It typically leverages a common human desire to want something "for free" in exchange for nothing more than a simple click. You can avoid a lot of misery with the simple concept think before you click. The entirety of Effective defenses against malware and other threats can be distilled into those four simple words.
It's also human to make mistakes. Adware distributors derive clickthrough revenue (💰) by inundating a Mac with advertising and directing search queries to sites specifically tailored to deliver it. If that's not bad enough, equally opportunistic scam artists sell products (💰) to eradicate the adware. Follow the money—it's as simple as that. If you should ever succumb to one of those scams, undoing the damage is easy enough, but no one should make such a mistake more than once.
Should that happen to you, removing adware is simple and does not require downloading or installing any product. On the other hand the eradication of "anti-virus" products and ridding a Mac of their effects is often cumbersome. "Webroot" happens to be one of the worst. In fact I tried and failed to uninstall it on one of my Macs, but eventually gave up and restored a Time Machine backup. That's how bad the "junk" can be—worse than the malware it claims to protect against.
