How are computer system viruses created?

Trojan horse are created by individuals who wish to trigger damage to others, or who wish to gain control over somebody else's computer system. The easiest way to develop an infection is to take some existing code and customize it a little.  https://thumbpower.org/?p=714https://islamhood.org/?p=585 This new code can then be distributed to others, who might unintentionally run it on their own computers. Once the infection is running, it can do anything its creator wants it to, including erasing files, stealing information, or sending out spam. More sophisticated infections can be really challenging to get rid of and spot.