USB drives are so popular and generally get used to move data between multiple systems frequently.Even though USB drives are very useful, they are potential breeding ground for viruses.

When you stick your flash drive the host computer will look for commands in "autorun.inf" to be executed automatically.Normally those commands in "autorun.inf" are harmless, but if the file was modified, it may call for execution of a Trojan or whatever program in your USB flash drive automatically.
Naevius USB Antivirus is a tool which protects your pen drive from autorun-viruses.