How do trojan horse spread out?

A virus is a type of harmful code or program that, when executed, duplicates itself by modifying other computer system programs and placing its own code. When this duplication prospers, the impacted areas are then said to be infected with a virus.

Viruses frequently spread by creating a copy of themselves on a user's computer system without the user's understanding or permission. In most cases, infections will be spread by e-mail accessories or by files shared on peer-to-peer networks. Once an infection has contaminated a computer, it can spread to other computer systems on the same network.

https://catbitcoin.com/?p=1498 Some infections are programmed to spread themselves in such a way that could trigger severe damage to a computer system. For example, a virus might erase files, fill a hard disk drive with worthless information, or corrupt system files. Sometimes, a virus may even show a message or image that is created to scare the user.

Many infections are reasonably harmless, and only trigger minor inconvenience to the user. Some viruses can be very unsafe, and can cause widespread damage to computer system systems.