"Because USB drives, sometimes known as thumb drives, are small, readily available, inexpensive, and extremely portable, they are popular for storing and transporting files from one computer to another. However, these same characteristics make them appealing to attackers.
One option is for attackers to use your USB drive to infect other computers. An attacker might infect a computer with malicious code, or malware, that can detect when a USB drive is plugged into a computer. The malware then downloads malicious code onto the drive. When the USB drive is plugged into another computer, the malware infects that computer." – Content courtesy of US-CERT Cyber Security Tip ST08-001 — Using Caution with USB Drives