A computer virus is a code or piece of software that when executed, can attach itself to other programs. Every time these programs run, the virus has a chance to replicate again and again, and so on. Viruses can cause complete destruction, by deleting programs or causing systems to crash, or be harmless, perhaps by flashing a ridiculous phrase on the computer screen. Viruses are intentional and are created by people. Viruses can be found as traditional viruses, email viruses, worms, or Trojan horses, among others.