But still really no longer than in the 21th century.

The percentage of people using antivirus software in the Windows environment before 2001 was absolutely puny.

We still have 25% of Windows users who disable their antivirus software.
And we have about 50% of uses who have postponed updating their antivirus software.

"Internet years" are way shorter than real years, making us very forgetful, and assuming conceptual changes happened way earlier than they really did. We might have been power users 10 years ago, but the knowledge gap to normal users was huge.

The point here is that Apple was basically the only computer company who did real work on virus-strengthening their systems during the 20th century, because of all problems with System 7. The Unix systems didn't had to do such work because of the lack of attacks and their inherrent sand-boxing. Microsoft didn't do any such work because they couldn't figure out a good way to do it without breaking the Win16/Win32 compatibility - the only reason for their huge market shares.

What we consider a widely spred "modern" or "educated" view on virus have happend during _this_ century. Only a very small percentage of users and some caring corporations did bother with antivirus software before 2001. And the antivirus software did not have any help from the operating systems, requiring them to slow down the computers to crawls because they couldn't just filter before entry. They had to filter everything on every access. And the antivirus software did not contain algorithmic scans so every slight permutation of a virus meant an instant need for an update. And already 2000 the availability of good Internet access was way worse than now. Who wanted to connect their modems and have the first minutes spent retrieving the nightly updates to the antivirus software?

A number of antivirus manufacturers had products 20+ years ago. But the fact is that the number of users were few. And the number of users bothered enough to also keep the software updated whas even less. The mindset simply was not there. "It doesn't happen to me" was the general view. It's the very widespread "true" broadband explosion that have made it the 21st century that have made the masses aware. We don't dial in to internet. We are constantly connected. And with Win 3.1 and in some part Win95/Win2k we did know a bit about the internals of the OS. With WinXP/Vista/7 there is too much black magic services doing things we don't know about and can't control. It's the 21st century that have seen the OS manufacturers do significant work - with basically Apple as the single exception in their work to replace System 7.