Virus/Spam spreads by removable disks ,they are best friends of viruses who likes to spread them all around computers network.
If you take good care of your computer against antivirus, chances are you will never get your USB drive infected also but this cannot be guaranteed if one of your friend is getting his USB drive to copy songs from your computer or you have used your USB drive in a cyber cafe. The only solution to this problem is always scan your USB drive before copying any file from it.
Though most of the antivirus these days have scan on plug feature but if for some reason it fails to detect your computer will be infected in no time.
Bit Defender recently launched a new tool against this kind of infection, USB Immunizer. This small tool stops any program from launching itself automatically when you plugin a USB.
Secondly, you can carry this program in your USB drive. So if you connect to a friends PC or any cyber cafe, lunch and keep it running so none of the malware or virus can copy themselves to your USB. You can even download it from any compute before using your USB if you are connected to internet and then use the drive.
Also make sure the switch to Immunize your computer is turned on. This will make sure no program launches automatically from USB drive.
Similar tool in this line is USB Security utilities which provides even more features like Automatic Backup, Built in Virus Scanner, Protection from Autorun and scan provides a risk level for each file.