How do trojan horse spread out?

A bug is a kind of malicious code or program that, when carried out, duplicates itself by modifying other computer system programs and inserting its own code.  https://gulfoilspillrecovery.org/?p=1371 When this duplication is successful, the impacted locations are then said to be contaminated with a virus.

Infections frequently spread by creating a copy of themselves on a user's computer without the user's understanding or authorization. In numerous cases, infections will be spread by email accessories or by files shared on peer-to-peer networks. Once a virus has infected a computer, it can infect other computer systems on the very same network.

Some viruses are set to spread themselves in a manner that might cause major damage to a computer system. For example, a virus might erase files, fill a hard disk drive with useless data, or corrupt system files. In some cases, a virus might even display a message or image that is created to frighten the user.

Most viruses are relatively safe, and just cause small annoyance to the user. However, some viruses can be very dangerous, and can cause extensive damage to computer systems.