Just to clarify:
Some porn websites will say that you need a video codec to view something and if you install their (fake) codec, you are really installing a trojan. Not a true self-replicating "virus". Not harmful, easy to remove. Not all that scary.
If you try to pirate (steal) copies of iLife or iWork apps, you may be really installing a trojan. Not a true self-replicating "virus". Not harmful, easy to remove. Not all that scary.
If you do a Google search for images and one of them is targeted by the MacDefender malware (also a trojan), you could receive a
fake alert that you are infected and then prompted to purchase an app to remove something that isn't even on your Mac in the first place. Don't give them your credit card # and follow
this advice to make sure it wasn't installed. Also not a true self-replicating "virus". Not harmful, easy to remove. Not all that scary.
Some members on here use
clamxav to monitor the risk of malware on their Macs. Probably not necessary, but if you are one of those that likes that extra "security blanket" to make you feel better, you might want to look into it.
Most members on here don't use any a/v software. Keep informed and don't install anything if you don't know what it is.