A Guide to the Main 6 Types of Computer Viruses

In much the same way that different human viruses cause different symptoms, there are different types of computer viruses that do different types of damage to computer systems and networks. It’s crucial for decision-makers and IT departments to understand these viruses and their potentially damaging impacts; both economic and operational. This article describes six main types of computer viruses and their effects.

What is a Computer Virus?

A computer virus is a piece of malicious software that can replicate itself inside a host application and spread across computers and networks when a user executes the host software. The mechanism is similar to the way in which a virus spreads through a human population (effective self-replication) by attaching itself to host humans.

Computer viruses can cause devastating financial losses. The fast-spreading MyDoom e-mail virus caused more than $38 billion worth of damage as far back as 2004. Computer viruses remain ever-present threats in the landscape of information security.

Computer Virus Symptoms

Sometimes the symptoms of a computer virus infection will be obvious, and sometimes they require a keen eye. Here are the most common signs and symptoms to look out for:

• Annoying messages appearing on your screen
• Complete hard drive shutdown
• System slowdowns due to the virus consuming system resources
• Important system files disappearing
• The size of installed programs increasing due to the virus replicating itself with those programs
• Systems randomly switching off or crashing
• Unexpected error messages and blue screens of death

The most effective and rapidly spreading viruses are often e-mail viruses. The virus might infiltrate email programs on an infected system and attach itself to any outgoing emails. This makes for a rapid way for a computer virus to replicate itself on other systems as unknowing users open email attachments and infect their computers.

Types of Computer Virus

The six broad types of computer viruses are:

1. Polymorphic viruses
2. Stealth viruses
3. Macro virus
4. Companion virus
5. Multipartite virus
6. Retrovirus

Polymorphic Viruses

A polymorphic computer virus gets its name from its ability to change itself into different forms to avoid detection. Polymorphic viruses often use cryptography methods to encrypt themselves and evade detection by antivirus software scanners. This is known as a viral mutation and it is similar to how biological viruses mutate their genes to avoid detection by host organisms.

Stealth Viruses

Stealth viruses, as their name suggests, are adept at avoiding detection by hiding in files, boot sectors, or hard disk partitions. Surprisingly, some leading technology blogs conflate stealth viruses and polymorphic viruses, but their mechanisms are not the same.

Stealth viruses are able to evade detection by redirecting commands around themselves. For example, a stealth virus in the boot sector of a drive will redirect any attempt to read the infiltrated sector of that drive

Macro Virus

A macro virus exploits coding languages known as macro languages that many users of Microsoft Excel and MS Word are familiar with. In fact, within an enterprise environment, it’s likely that many employees use macros several times in their daily work. Macros are simple pieces of code composed of repeated actions that people create to save time. For example, you might create a macro to automatically hide all worksheets in Excel except the active sheet.

The macro language for a particular software is embedded in the software itself. A macro virus is written in the same macro language as the application it infects. The macro virus typically uses the same name as a normal macro and replaces its commands with malicious tasks, such as corrupting systems.

Companion Virus

Companion computer viruses tricks users by using the same naming convention as commonly executed software. The companion virus uses a different extension than the executable file. When the user attempts to run the commonly used program, the companion virus executes instead. Companion viruses can be tricky to spot because unlike many other types of computer viruses, they don’t modify any files.

Multipartite Virus

A multipartite virus attacks both executable files and boot sectors on a system in order to increase its ability to infiltrate a system and spread. A key indicator of infiltration by a multipartite virus is if a system becomes low on virtual memory. Multipartite viruses can be extremely destructive and difficult to completely eliminate. Even if you clear the virus from all program files on a system, its multi-pronged approach means it can easily infect the system again during the bootup process.

Retrovirus

A retrovirus is a unique type of computer virus that actively seeks out anti-virus software and attempts to disable any virus-scanning functionality. A retrovirus can destroy virus definition files, rendering the antivirus software useless. Retroviruses can also directly attack antivirus software and create exceptions allowing them to spread on a system.

The Importance of Antivirus Software

One of the main ways for businesses and individuals to protect against viruses is to use effective antivirus software The efficacy of antivirus software depends on its definition database containing known antivirus signatures. The database should be updated frequently to reflect new viruses and it should be extensive enough to encompass all known viruses.

The utility of good antivirus software is that it offers comprehensive protection against all types of computer viruses. It’s also worth remembering that a virus can destroy a computer system or multiple systems on a network. The cost of an effective antivirus program is far less than the cost of replacing hardware rendered useless by a really malicious computer virus.

Employee education is also important. People should know about common vectors of computer virus attacks and prevent them. For example, never opening email attachments from untrusted sources.

 

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