Are Mac's and Apple's and such Really Virus-free?

Hey guys, I recently (1-2 months) acquired a Mac PowerBook G4. I've heard Loads of rumors about Mac's being the "Invincible Computers", and there are only like 5 known viruses for the Mac. Is this true? And does it pertain to Malware/Spyware/AdWare too? Gimme some freedback. Much appreciated.