I have had various Macs now for the best part of 5 years, I also know of at least half a dozen other Mac users. I have never heard of anyone getting a virus. Now that is not to say it can't happen but........
At the moment we run three Macs in the house all on pretty well all day and with no problems. (Touching wood!)
What I do ensure is that my Firewall is on. (System Preferences>Internet & Network>Sharing>Firewall).
I also spent some time and extra money when I bought my wireless router to try and get the one with most inbuilt "firewalls".
I am sure the Mac experts will be able to expand on this later on, I am just a humble user.