How Do Computer Viruses Spread?

So you've caught a computer virus--how did you get it? Even if your anti-virus software cleans it up, it may not always be clear exactly where it came from. Computers were not always networked via the Internet. Back then, computer viruses would spread much more slowly. Most often viruses spread through the use of the most common form of removable media: floppy disks. (Many of you may never have seen a floppy disk.) Once computers became frequently networked via bulletin board systems (BBS), ARPANET, and later the Internet, viruses were able to spread much more quickly. Let's examine how they have spread and do spread.

The following are not the only possible means of infection, but are some common and likely ways you can be infected:

Web pages - Either via a download, web code, or a cross-site scripting (CSS) attack, web pages can be a source of viral infection. As browsers gain more features and functions more methods for virus propagation appear.

Messages - E-mail messages as well as IM or chat messages can be a vector for viral infection. Whether the virus is spread through links, embedded code, or an attachment depends on the type of message and the client software.

Infected programs or Trojans - Running an infected program that you copied from a CD or other media (or downloaded) is a means of infection. Sometimes the virus is a Trojan, where it appears to be a useful program, but is in fact a virus.

Direct connection and infection by worms - Worms can connect directly to a computer across the network, and if the target computer is vulnerable, the worm will infect the target. This is notably different since no user action has to occur.

If your anti-virus software or other protection software failed, the virus can probably spread. How it spreads depends on the kind of virus, of course. The virus might use your e-mail or IM contact list and send messages to your friends, family, and other contacts containing the virus as an executable attachment, when they open the attachment they might be infected if their anti-virus software doesn't stop the virus. The virus you have might spread that way even if you didn't catch it the same way.

Interestingly, network firewalls are often more permissive for outbound connections, so that viruses or worms which make network connections may be allowed to do so without being stopped. Most good host-based firewall software and the Windows built-in firewall software can be configured to block outbound access and ask you if an unauthorized program should be allowed to access the Internet. This method isn't foolproof, of course.

The virus could also infect removable media that you access or create, so if you send or share that media others could be infected. If you have been infected and have cleaned your computer and remove the virus, be sure to scan any media you have created during the time you were infected.

Check out my articles on the different kinds of computer viruses, how computer viruses are made, what computer worms are, and the differences between worms and viruses.