Although Mac malware is almost non-existent, that is no reason to become complacent and think the Mac is completely immune. A trojan named Flashback with several variants that supports remote control of a Mac has infected over 600,000 Macs by exploiting a vulnerability in Java. It can be activated by inadvertently visiting certain web sites that then trigger what looks like a Flash software update.
To be safe you should do 2 things:
1. Download and install the Mac system update for Java to protect against any future infections by this trojan
2. Enter at least a couple of terminal commands to determine if you have already been infected.
This TidBITS link provides the default read terminal commands to detect the infection and instructions for its removal if you have already been affected. Note that this trojan works in stealth mode so you might not see any symptoms even if you are infected. Do the tests just for peace of mind that your computer isn’t part of a botnet.