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.
Hoaxes are false alerts about viruses that don't exist. For a list of hoaxes check out Hoax Warnings On The Run. Another good site to visit is Computer Virus Myths. We highly recommend that you have Virus Protection installed in your computer before you consider downloading anything. This site will assist you fight the virus war with software, an encyclopedia, primers, alerts and research.