How are computer viruses developed?

Virus are created by people who wish to trigger harm to others, or who wish to get control over someone else's computer system. The easiest method to produce a virus is to take some existing code and modify it a little. This new code can then be distributed to others, who may unwittingly run it on their own computer systems. When the infection is running, it can do anything its developer wants it to, including erasing files, taking data, or sending spam.  see this page More advanced infections can be really hard to discover and get rid of.