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Presuming you are already familiar with what a computer virus is, how it spreads, and what it does, it is time for you to learn that there are a few different types of computer virus. Let’s see what these types are and how each of these computer viruses behaves.
Boot viruses
Since nobody uses floppy disks anymore, these types of computer virus are left to infect only the master boot records of the hard disk. The boot record program loads the operating system in the memory at startup. These viruses replace the boot record and move it to a different part of the hard disk, or simply overwrite it. As you can imagine, once you start your operating system, it loads into memory, along with the virus. Once the virus is in the memory, it is free to do whatever it was programmed to do. To avoid loading the boot record viruses into memory, you must boot your operating system from another source (another hard drive or a bootable CD/DVD).
Program viruses
These types of computer virus will infect only executable files (with extensions like .BIN, .COM, .EXE, .OVL, .DRV, and .SYS). Once executed, these programs load into memory, along with the virus contained within them. Once in the memory, the scenario repeats – the virus is free to act and infect other files or simply deliver its payload. These viruses are friendlier than boot viruses and can be removed a lot easier.
Multipartite viruses
These computer viruses are hybrids, derived from boot viruses and program viruses. They infect executable files, just like the program viruses but, once the executable is executed, it infects the master boot records. The scenario is similar to the boot virus’s one: once you boot your operating system, the virus is loaded into the memory, from where it is free to infect other programs and replicate itself, ultimately delivering the payload.
Stealth viruses
These viruses are specialized in avoiding detection and will use a number of techniques to do so. Most of them simply redirect the hard disk head, forcing it to read another memory sector instead of their own. Some of them also alter the reading of the file size shown when listing the directory. These types of computer virus are very hard to find by humans, but antivirus software is specially designed to track them down and erase them.
Polymorphic viruses
The polymorphic viruses will always change their source code from one infection to another. Each infection is different, and this makes detection very hard. However, detection is still possible, depending on the antivirus.
Macro viruses
This virus is relatively new and it infects macros within a template or document. When you open a word processing document, it activates the virus. The virus infects the Normal.dot template, which is a general file used by all the documents. So, whenever you open an uninfected document, by referring to the Normal.dot file, it gets infected as well. This infection can only spread if infected documents are opened on another machine.
Active X viruses
Most people do not know how to configure ActiveX and Java controls, unconsciously leaving a security hole. Applets are then allowed to run freely on the machine, delivering all ActiveX viruses. By simply turning off some ActiveX and Java controls in the browser, a user can efficiently protect their PC from this type of computer virus.
