Click for more. A computer virus is a software program that attaches itself to, overwrites or otherwise replaces another program in order to reproduce itself without the knowledge of the computer user.
Most viruses are relatively harmless.
One could be present on your computer for years without your knowledge with no noticeable effect. Still others can cause unintended damage. Kinds of computer viruses Viruses have been given some unusual names. A dropper is a program which installs a virus or Trojan, often covertly. And a worm is a program which spreads, usually over network connections.