Yes... you could definitely get a virus.
Get anti-virus protection and scan all files with it before running or installing them.
Mac's aren't bullet proof. They just aren't as popular.
This is an old argument. Macs are not "bullet proof", but considering that there are no known OSX viruses loose on the Web, your vulnerability is extremely limited.
Scanning all files is not necessary, don't bring your Windows paranoia with you to the Mac experience. I have encountered many Windows viruses via e-mail and simply delete them via Secure Delete to make sure they are gone.
The argument about popularity vs System security has already been addressed, and I believe it is a bit of both. Apple is MUCH more proactive about system vulnerabilities, as they are exposed, than MS, who tends to be reactive.
All that being said, I have been running without an AV app running on my Mac for years and have never run into an issue. The ONLY real reason to invest in one is to scan e-mails to keep from passing nasties on to your PC using friends.