Virus on High Sierra cMP? Slowdown and Contacts records deleted.

[macmesser]

I have been running High Sierra happily on a cMP 5,1 (flashed) for years and things have worked fine except for self-caused problems which have always been resolved. Over the past several months, both Safari and Chrome have experienced slowdowns and freezes. The Finder also seems affected, as even after force quitting the browsers, the entire system sometimes gets sluggish and/or apparently non-responsive. Restarting generally helps, but the system gradually goes south again. This is all too frequent and impacting productivity considerably. A few days ago the OS updated and after rebooting seemed OK, but while I was adding some contacts things got slow, so I force quit everything and rebooted, only to find that all my contacts were gone (not just the ones I was adding, all older contacts were wiped out). The system seems to be running OK now. Does this sound like a virus? What files do I need to restore from Time Machine to get my contacts back? My Intego anti-virus found only a few routine problems.