help me understand how a Mac is virtually immune to viruses?

After my initial question about a computer switch to Mac, I got to thinking
about some of my colleagues at work mentioning that a Mac is virtually
immune to Win viruses. I know this may be elementary (my dear Watson), but
how is this so? Wouldn't the Mac be using the same programs for Win surfing
(Firefox, for example) and be receiving the same spyware and other crap
downloaded in the system as on the Win environment?

thanks,
Beryl