I just ran across an article regarding a new discovered security flaw in any USB devices. And yes Virginia, this could affect Mac's. Someone has discovered how to store viruses / malware in the firmware of a USB devices. Here is an excerpt from the article along with a link to the full article.
"Undetectable malware can be hidden in any USB flash drive, according to security researchers Karsten Nohl and Jakob Lell. This is very bad news for home users who pass around USB drives, and for corporate IT managers who may have to ban the popular devices from business networks
To demonstrate the vulnerability of USB drives, the researchers wrote some proof-of-concept malware (which we can only hope no one copies) called BadUSB. It is a collection of malicious apps that can modify any software installed from a USB drive on a target computer; completely take over control of an infected PC; and even redirect users’ Internet traffic.
Erasing or reformatting the USB drive does not destroy the malware, which hides in the USB device’s firmware that controls the drive’s basic functions. This previously unknown vulnerability is part of the USB standard’s design; as such, it can’t be eliminated without re-engineering every USB device."