The Different Kinds of Computer Viruses

The term computer virus is well known. There are different kinds of viruses however, and some of these types are less well known than others. Viruses have been around almost since the beginning of computers. Before almost all computers were networked, viruses would spread much more slowly. They often spread on floppy disks; it was important to scan the disks to ensure that viruses were not present. Viruses can spread much more quickly today with the near ubiquitous connectivity to the Internet.

There are several different kinds of computer viruses. The possible types have expanded as operating systems have grown in features and complexity. The same is true of software programs, scripting languages, and server applications. Programmers are trained these days to work to limit the possibilities of virus infection, and there are programming tools to assist in strengthening code. Still, viruses are prevalent. Let's look at the different kinds of viruses.

Boot sector viruses - Computer operating systems use part of the hard disk when starting up after the computer is turned on. This area of the hard disk is called the boot sector. When a virus infects the boot sector it can load itself before the operating system. When floppy disks (which also have boot sectors) were common they could be used to boot as well and were a common vector for infection.

Memory resident viruses - These viruses stay in memory, like the name suggests. Others just move from file to file and program and program.

Polymorphic viruses - These change their "shape" to make them harder for anti-virus software to detect.

Macro viruses - Many business software programs and suites have macro language and scripting features that can be converted to use for viruses.

CSS viruses - Browsers that have scripting functionality enabled can be exploited by cross-site scripting. Web sites with advertisements that come from another source (as most do) can be especially vulnerable.

Cavity viruses - These viruses hide within executable files using the "free space" within the files so that the total file size does not change and arouse suspicion.

Metamorphic viruses - these rewrite themselves each time they infect a new file.

Trojans - A trojan appears to be one type of file or program but is actually a virus.

Worms - Worms do not require user actions to spread, they move from system to system over networks on their own. For more information see my articles on computer worms, specifically the Conficker worm, and how to protect your computers and networks from them.

The best defense is to ensure you have anti-virus software installed and keep it up to date. Most modern anti-virus software automatically updates, but don't take it for granted that this is happening. Many personal firewalls will also provide protection against viruses and against their spread. I hope this article has broadened your view of computer viruses.

For more information on this topic, check out my articles on the differences between computer worms and viruses, and what is a bloodhound virus? Also learn about how computer viruses spread, and how people make computer viruses.

For more help staying safe online, check out my articles The Top 5 Security Mistakes People Make when Browsing the Web, and The Top Five Security Mistakes Users Keep Making.