How do trojan horse spread out?

A bug is a kind of harmful code or program that, when carried out, duplicates itself by modifying other computer system programs and placing its own code. When this replication prospers, the affected areas are then said to be infected with a virus.

Infections often spread out by creating a copy of themselves on a user's computer without the user's knowledge or consent. In most cases, infections will be spread by email attachments or by files shared on peer-to-peer networks.  https://9tolife.co.uk/?p=669 Once an infection has contaminated a computer system, it can spread out to other computer systems on the very same network.

Some viruses are programmed to spread themselves in a manner that could trigger serious damage to a computer system. An infection might delete files, fill a tough drive with useless information, or corrupt system files. In some cases, a virus may even display a message or image that is created to scare the user.

Most viruses are reasonably harmless, and only trigger small annoyance to the user. Nevertheless, some infections can be very hazardous, and can trigger extensive damage to computer systems.