The seriousness of pen-drive viruses and their ignorance: Simple steps to protect yourself

I have seen to many people ignorant of the fact that their pen-drives (usb key, usb disk) have been contaminated by viruses that have been copied by them unknowingly! How? And why didn’t their antivirus detect them? Some do not even know until their computers start becoming slow and more… Let me explain easy ways to detect and protect yourself…