The Mac environment wasn’t always virus-free.
Once upon a time, I ran a few public-access labs at my university.
These labs suffered several virus outbreaks, most notably catching
WDEF before it was discovered in 1989. We also would occasionally
see MDEF, nVIR, and Scores. The Mac’s market share then was a
little higher — a little under 10 percent, versus 5 or 6 percent
today — but it was still a minority platform, with the great
majority of computers running DOS or Windows.