Constantly Backing Up

I know of the option to back up your data on an external hard drive, at intervals, but in the past few years I've heard of "constantly backing up" which is basically that whenever something is downloaded on your computer, an exact copy is made on the external drive. All downloaded programs/applications on the computer are also reflected on the external drive, so I guess this is what imaging is all about.

I understand this is so that if you're computer gets into an unrecoverable state, the backup drive would be there to the rescue. Now my question is, if your computer screws up through an update, virus, or the like...wouldn't the external drive get affected in the same way? It's constantly connected to your computer and reflecting everything from it onto itself, so wouldn't any malfunction or malware creep into it and screw it up as well?

Yes, it's possible to get continuous data protection (CDP) which backs up changes made to the drive, allowing you to recover to any point in time (assuming you've got the backup data for this). So, if you installed an app you didn't like, it would be possible to roll back to before this happened.

I'm not sure what real time backup / CDP options exist for home PCs, but perhaps someone else will have more experience and can add to this.