The word 'slowly' is what defines the thread for me.
On topic, my office-provided Macbook Pro has had antivirus installed from day 1, it was Sophos, I think.
It traps about one infection per week, and though a little unnecessary, it also traps malware such as URL:Mal that has no effect on MacOSX at all.
Malware for Macs are about as old as Macs themselves. I do remember the 4th generation RISC CPU had a few viruses written for it, but the writer didn't bother to update the code because there weren't enough machines to infect.
Those were the days Apple laptops came with a B&W screen and a trackball: