Computer viruses are pieces of software code that attach themselves to other software. When the host program runs they replicate themselves and spread to other programs.

By analogy with biological viruses, they insert themselves into other programs, just like biological viruses to do to cells, and have the other program execute them.

These were the original computer threat, and have been around as long as programs have. They never spread beyond where they can see other programs, so the only way to get thems between computers is sharing files over the network or moving them around manually.

Computer viruses were famous for doing all sorts of weird things to your computer. Some of them even practiced extortion by holding your data hostage until you typed a password, and other such silliness. The very first virus, Elk Cloner, was a practical joke, and others have followed in those footsteps by merely popping up messages.

They can sneak into you computer from any outside source and the only way to protect your computer is using an Anti virus software such as Norton Antivirus or Norton Internet Security.