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As we've seen, once a virus is activated on a computer system, it knows how to locate and infect host programs on that machine. To replicate within the system, a virus might attach to boot sectors of floppy disks and hard drives. It might also look for documents, executables, or scripts in which it can embed its code. To be in a position to continuously infect new files, a virus can even load itself into memory or into a template document. However, at some point, a virus confined to a single box will run out of new host programs to infect. To reach its replication potential, a virus needs to be able to copy itself to new systems that contain targets not yet infected.
Unlike worms, pure viruses cannot propagate autonomously across the network—they require human help to move from one machine to another. In this section, we'll look at some of the ways in which viruses reach new systems through the use of removable storage, e-mail and downloads, and shared directories.
Removable Storage
When Apple released the first iMac in 1998, many were bewildered to learn that the company had no plans to include a floppy disk drive with the new system. At the time, this approach seemed impractical. After all, floppies had become a seemingly permanent fixture in personal computing, and were used as the primary device for sharing documents and other files until networks and writable CDs became affordable and ubiquitous. Although not used much now, floppy disks had been with us since the dawn of computer viruses.
The authors of early viruses such as Elk Cloner realized that they could take advantage of people's tendency to share removable media, and were able to spread their creations by infecting boot sectors of floppy disks. This trend continued well into the 1990s, when boot sector infectors comprised a significant proportion of the virus population. Because of the popularity of viruses that targeted boot sectors, many antivirus programs still warn you if you are shutting down a system while a floppy disk is inserted into its drive. This alert is meant to prevent you from inadvertently booting the machine next time using a floppy that has malicious code embedded into its boot sector.
Boot sector viruses have traditionally relied on floppy disks for propagating across systems. Theoretically, a virus could also target a boot sector on a CD-ROM. In practice, though, a virus can rarely rely on the ability to attach to the CD's boot sector, because CD-ROMs are not writable once they have been mastered. Even writable CD media such as CD-R and CD-RW are not practical targets for boot sector infectors because this media type is not modifiable once the user creates the CD and closes the session. This same reasoning applies to DVD-based media.
Besides boot sector infectors, viruses that target executable files and scripts also can use removable media for moving across systems. The user is expected to save the infected file onto a floppy or a writable CD, and then transport the virus on the removable media to another victim's computer. Although end users unwittingly do their part in distributing infected files through these mechanisms, some software vendors also have been known to accidentally ship media that contained malware to their customers. For instance, a copy of the CIH (also known as Chernobyl) virus was included in Yamaha's CD-R drive firmware update, and also resided on a CD distributed by several gaming magazines.
Although using floppies to share files is no longer in fashion, we continue to exchange documents using removable media. Writable CDs are sufficiently inexpensive that we don't think twice about burning some files onto them and passing them out like candy, and writable DVD media are heading in the same direction. Other types of removable storage devices that have gained significant popularity are USB keychain drives and flash media such as SecureDigital and CompactFlash cards. As long as people continue to exchange files through such removable media, viruses will have a way to spread from one system to another. You should be on the lookout for victims transporting infected files on USB keychain drives.
E-Mail and Downloads
Of course, there is a way to share files without relying on removable media. E-mail is one of the most convenient and popular ways of exchanging information. Although the body of a plain text message cannot carry executable code, its attachments surely can. An unsuspecting user can e-mail an infected document to a colleague or a friend even more easily than by using a floppy disk.
The most memorable malware outbreaks associated with the use of e-mail attachments have been those that involve automated techniques in which malicious code e-mails itself to potential victims.
Viruses can also get into our networks through the files that we download from Web sites or newsgroups. The Melissa virus, for example, is believed to have entered the world through a posting to the alt.sex newsgroup that contained a file called List.doc. Similarly, any executable or a document obtained from a remote Web server might be infected with a virus. Download the file, run it, and you've just inadvertently invited a virus onto your system.
Shared Directories
Yet another way in which people assist viruses in reaching new systems is by storing infected files in shared directories. Furthermore, the same techniques that viruses use to traverse directories on a local system can allow them to seek out and infect files located on shared directories that are located on a file server. Various file-sharing mechanisms could propagate viruses, including Windows file sharing via the Server Message Block (SMB) protocol, Network File System (NFS) shares, or even peer-to-peer services like Gnutella, Kazaa, and Morpheus.
A multiuser file server is a prime location for malware because there is a good chance that one user's document or program saved to a shared directory will be accessed by another user coming from a different PC. The file server acts as a common infection point, where various machines exchange virus-contaminated files. Conveniently, such centralized storage mechanisms also provide us, the defenders, with the ability to detect and eliminate known viruses in one shot by scanning the server with antivirus software. | 
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