Most of the common Mac viruses were executable infectors. They spread (past tense) through software sharing -- running an infected application would infect your system, causing it to spread the infection to any applications you launched.
This lifecycle depended on having some way for those infected executables to get run by uninfected users. That was where personal software trading came into the picture.
Anyways, as more users started downloading software instead of trading it, that cycle broke. The big shareware sites like Info-Mac would only accepted software from authors, not from users, and would run virus scans on anything they published anyways.
No so much, software was generally obtained straight from the vendor's site, often legally. Although piracy existed, there was some sort of honor culture around the Mac.
This lifecycle depended on having some way for those infected executables to get run by uninfected users. That was where personal software trading came into the picture.
Anyways, as more users started downloading software instead of trading it, that cycle broke. The big shareware sites like Info-Mac would only accepted software from authors, not from users, and would run virus scans on anything they published anyways.