Viruses behave in different ways. Some viruses stay active in memory until you turn off your computer. Other viruses stay active only as long as the infected applications is running. Turning off your computer or exiting the application removes the virus from memory, but does not remove the virus from the infected file or disk. Some viruses are programmed specifically to damage the data on your computer by corrupting programs, deleting files, or erasing your entire hard disk.
All computer viruses are manmade.
A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. A trojan horse is a program that appears to serve some useful purpose or provide entertainment, which encourages you to run it. But, like the Trojan horse of old, it also serves a covert purpose which may be to damage files or perhaps plant a virus on your computer. Many computer viruses turn out to be hoaxes or myths.