A Trojan Horse malware called Flashback has infected an estimated hundreds of thousands of Macs. If you've stayed away from bad neighborhoods on the InterWebs, you're probably OK, but just to be safe, Check out this link. It has easy-to-follow steps to detect and remove the malware.
If you are running Snow Leopard or Lion, run Software Update to get the latest version of Java, which patches the vulnerability that Flashback exploits.
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