Computer viruses are sets of code, or instructions, written in computer-language.
This is how the Virus spreads. A few people in a company may have shared disks, but the virus would usually stay in one place and be detected before many computers were harmed. Now people use the Internet to share files, send emails and work on programs together. This means a virus can spread from computer to computer all over the world before it is caught. Viruses can act in different ways, from simply making the letter E disappear off your screen, to destroying all the memory in your hard drive.