A virus is a program or piece of code that is loaded onto your computer and runs without your knowledge. Some are simply annoying, while others can be very destructive.
Most viruses replicate themselves.
All computer viruses are manmade. Some people distinguish between general viruses and worms. Most worms will try to infect as many computer files as possible, including free space. Other worms duplicate by attaching themselves to e-mail, or other file transfers. A macro virus is embedded in a document instead of a program.