You're basically asking the question "If my hard drive had different data, what might happen?" Unfortunately, this is a question that's basically impossible to answer specifically without knowing what data changed.
Imagine that somehow the drive was damaged in a way that modified data on the drive to coincidentally be equivalent to a virus. You'd now have a virus on your machine. On the other hand, that modified data could also just create a text file. It's entirely dependent on how the data changes.
Of course, both of these scenarios are extremely unlikely. Far more likely, you'll find some piece of your hard drive unreadable and lose some files or something.
So how will you know if something happens?
Check for SMART errors. SMART is a monitoring system that detects and reports on various indicators of drive reliability. If something goes wrong with the drive, SMART likely will report something wrong, and macOS will notify you. Of course, SMART isn't perfect, but it does help.
Additionally, if something appears to be off, you can run Disk Utility's first aid, which will scan for and attempt to correct errors. Sadly, this system isn't perfect, and the HFS+ file system that macOS uses is fairly susceptible to bit rot, which is a form of data loss.
In general, the best strategy for preventing data loss is to keep backups. As for security: well, I wouldn't worry about it.