How do viruses spread?

The most widely spread viruses are those having worm capabilities (VBS/Loveletter, W97M/Melissa.A@MM). The worms are spreading rapidly, too: VBS/Stages, I_Worm/Kak, I_Worm/ZipExplorer or I_Worm/Happy99. The expansion of the Internet put them on the first place regarding virus spreading.
The exchange of documents between users is a favorable way of spreading macro viruses.
In the last years, boot viruses lost their popularity because the floppy disks are more and more rarely used.

When you execute program code that's infected by a virus, the virus code will also run and try to infect other programs, either on the same computer or on other computers connected to it over a network . And the newly infected programs will try to infect yet more programs.

When you share a copy of an infected file with other computer users, running the file may also infect their computers; and files from those computers may spread the infection to yet more computers.

If your computer is infected with a boot sector virus, the virus tries to write copies of itself to the system areas of floppy disks and hard disks. Then the infected floppy disks may infect other computers that boot from them, and the virus copy on the hard disk will try to infect still more floppies.

Some viruses, known as 'multipartite' viruses, can spread both by infecting files and by infecting the boot areas of floppy disks.

What kind of files can spread viruses?

Viruses have the potential to infect any type of executable code, not just the files that are commonly called 'program files'. For example, some viruses infect executable code in the boot sector of floppy disks or in system areas of hard drives. Another type of virus, known as a 'macro' virus, can infect word processing and spreadsheet documents that use macros. And it's possible for HTML documents containing JavaScript or other types of executable code to spread viruses or other malicious code.

Since virus code must be executed to have any effect, files that the computer treats as pure data are safe. This includes graphics and sound files such as .gif, .jpg, .mp3, .wav, etc., as well as plain text in .txt files. For example, just viewing picture files won't infect your computer with a virus. The virus code has to be in a form, such as an .exe program file or a Word .doc file, that the computer will actually try to execute.

What's the story on viruses and E-mail?

You can't get a virus just by reading a plain-text E-mail message or Usenet post. What you have to watch out for are encoded messages containing embedded executable code (i.e., JavaScript in an HTML message) or messages that include an executable file attachment (i.e., an encoded program file or a Word document containing macros).

In order to activate a virus or Trojan horse program, your computer has to execute some type of code. This could be a program attached to an E-mail, a Word document you downloaded from the Internet, or something received on a floppy disk. There's no special hazard in files attached to Usenet posts or E-mail messages: they're no more dangerous than any other file.

How do I know if my computer has a virus?

The trigger that activates the virus can be almost anything. For instance the virus can be activated the minute it is installed. Or it may start its dirty work the next time you start your computer. In many cases, a virus can reside inside your computer in an inactive state, waiting for a certain event to happen. Like having a physical virus in your body, you can have a virus in your computer system and not even know it. A virus can wait patiently in your system and attack only on a certain date. Of course, in the time between the moment that you contracted the virus and the time that it made itself know to you, you may have innocently spread the virus to others.