Viruses are spread when you launch an infected application or start your computer from a disk that has infected file system. Some viruses spread themselves by using security flaws in e-mail applications, emailing themselves out to people listed in the address book of the person with the infected files. Other viruses will infect any floppy disk written to by the infected system. It is not possible to spread a virus without downloading something onto another system. Computer viruses are not applications suddenly turned evil or gone terribly awry.
People make all viruses.
You can get a virus from your mother, father, brother, boss, or anyone else who gives you an infected program, and everyone would be oblivious to this fact until it was too late. First off, a Trojan horses is not virus. A Trojan horse is a program that appears, on the outside, to serve a useful purpose, provide entertainment, or some other facet which encourages the user to run it. But, like it's name implies, it will usually release a payload that can damage files or perhaps plant a virus on your computer. Many computer viruses turn out to be hoaxes or myths.