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A Trojan horse program is named after the infamous Greek Trojan horse of history. A Trojan or Trojan horse is an imposter file, presented to you as doing something beneficial or benign to your computer. When run, the Trojan might or might not do what it claims, but it will also or instead will do something else, such as format your hard-drive. A Trojan does not replicate. A computer virus is a small program written to do something to a computer without the consent (and often the knowledge) of the computer user. For a program to be considered to be a virus, it must somehow cause itself to be replicated once it has been executed by the user. So, for e-mail attachments, a virus file would need to be double-clicked to activate and then replicate on that computer. A worm is a program which actively replicates itself to other computers. A worm arriving via e-mail would start spreading itself to other computers as soon as it was activated. This could be by sending itself out via e-mail or by exploiting open file shares on a network. These days, many examples of malware combine the techniques into hybrids, and debating the differences becomes an exercise for purists.
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