But they hade their largest problems with virus around 1990 (Mac Classic, Mac Plus, ...), after which they did a good catchup.

After that, it's the user base that have forgotten about all the early Mac virus problems, and being so used to claim viruses only a problem for Windows.

And next thing - Microsoft did not confront this problem until just a few years ago, by being restrictive with access rights and auto-install, and by developing their own malware removal tools.

So the 20th century was a time when Mac was the only brand (I'm ignoring Amiga and other game computers where people duplicated games like crazy and installed whatever they got their hands on) that had been suffering from virus and had done some work to solve it. M$ had not taken their time to try to close down their leaky bucket. And the different Unix versions had too little market shares while at the same time had a multiuser design (and without any auto-install features) where each user had limited access rights. So they hadn't had any need to do any antivirus work just for the reason the few virus that existed normally just hurt the single user who was stupid enough to install what he/she shoudlnt'.

So no, I can't really see how the 20th century was a time when everybody else confroted the problem and learned to deal with it.