Many Mac users are glad they don't have to worry about viruses, trojan horses and other nasty things from the Internet that PC users have to deal with.
They are partially right, but mostly they are just lucky if they haven't been attacked even though they don't run anti-virus and firewall software. All computers that let users install new software are vulnerable to malware, and the Mac OS isn't inherently safer than other operating systems. Apple computers have been a small part of the computer market (some estimates at about 5%) for many years. The people that write viruses want to affect as many computers as they can so they usually target computers running some version of Microsoft Windows.
As Macs become increasingly popular they have become more attractive targets for the virus writers too, so if you aren't already running anti-virus and firewall software on your Mac you should start right away. Take a look at the Sophos site for a list of the newer viruses and trojan horses designed just for Mac users, and be very careful when downloading music, games, videos or any files from less legitimate websites.