New information came to light recently regarding Fruitfly, also detected by some antivirus systems as Quimitchin, which was discovered quietly infecting machines in January 2017. An unusual Mac-specific APT that later was found to have Windows variants (PDF), Fruitfly was able to capture screenshots, keystrokes, images from the webcam and system information from infected machines. At that time it was believed it was at most a decade old, placing the earliest possible infections in that timeline around 2007 and thus after the Intel transition. The author, 28-year-old Phillip Durachinsky, was eventually charged in January of this year with various crimes linked to its creation and deployment.
Late last month, however, court documents demonstrated that Durachinsky actually created the first versions of Fruitfly when he was 14 years old, i.e., 2003. This indicates there must be a PowerPC-compatible variant which can infect systems going back to at least Panther and probably Jaguar, and squares well with contemporary analyses that found Fruitfly had "ancient" system calls in its code, including, incredibly, GWorld and QuickDraw ones.
The history the FBI relates suggests that early infections were initiated manually by him, largely for the purpose of catching compromising webcam pictures and intercepting screenshots and logins when users entered keystrokes suggesting sexual content. If you have an iSight with the iris closed, though, there was no way he could trigger that because of the hardware cutoff, another benefit of having an actual switch on our computer cameras (except the iMac G5, which was a bag of hurt anyway and one of the few Power Macs I don't care for).
Fruitfly spreads by attacking weak passwords for AFP (Apple Filing Protocol) servers, as well as RDP, VNC, SSH and (on later Macs) Back to My Mac. Fortunately, however, it doesn't seem to get its hooks very deep into the OS. It can be relatively easily found by looking for a suspicious launch agent in ~/Library/LaunchAgents (a Power Mac would undoubtedly be affected by variant A, so check ~/Library/LaunchAgents/com.client.client.plist first), and if this file is present, launchctl unload it, delete it, and delete either ~/.client or ~/fpsaud depending on the variant the system was infected with. After that, change all your passwords and make sure you're not exposing those services where you oughtn't anymore!
For the very early pre-Tiger versions, however, assuming they exist, no one seems to know how currently those might have been kicked off because those systems lack launchd. It's possible it could have insinuated itself as a login item or into the system startup scripts, or potentially the Library/StartupItems folder, but it's probable we'll never know for sure because whatever infected systems dated from that time period have either been junked or paved over. Nevertheless, if you find a file named ~/.client on your system regardless of version that you didn't put there, assume you are infected and proceed accordingly.