As a Mac user you don't have much to worry about for a couple of simple reasons:

1. Macs comprise less than 10% of computers worldwide, Windows 90%. If you were going to write a virus or a worm or a trojan or spyware which would you write the program for? It's a no brainer.

The likelihood of you getting hit on an Apple is insignificant compared to PCs

2. mac OS X is based on Unix, which benefits from decades of unhurried and thorough development for academic and scientific environments where security has always been important.

It is, however, possible for an attacker to target your Mac if you are directly connected to the internet and you are not careful (your built-in Mac firewall needs to be secure and you need to use 'proper' passwords ). There are legions of unsecured PCs out there that are infected with code that allows them to be controlled by spammers or worse.

The best way for the average user to remain secure is to put a router between your computer and the net. If you are sharing your broadband connection in the office you must already be doing this and are thus employing NAT, Network Address Translation, which makes the computers on your network relatively invisible to malicious scans.

You can, however, still download a virus attached to an email or from a dodgy web site. Because the virus is usually written for a PC it probably won't affect your Mac but it will affect the Windows PC that you unwittingly pass it on to. Not a good way to keep your clients happy.

So it's a good idea to scan for the odd virus using ClamXav, a good open source, and therefore free, scanner that automatically keeps itself up to date as new virus definitions are released. Once you locate a virus you can erase it. If it's in a document you need to keep you have a problem. You will need to buy a commercial product to attempt cleaning the document. The top rating programs are Intego Virus Barrier (my choice) or Norton AntiVirus10.x.x.

Disgruntled employees or ex-employees are also a threat. It's a pain but you need to allow for this possibility.

If your curious about how an attacker goes about their business you can read more here.

All of the above applies to your average situations, if you are dealing with sensitive information you you might consider seeking expert advice.