The Different Types Of Viruses

There are many different types of viruses. I will be looking at Overwriting, Appeneder, Prepender and EPO Viruses.

Overwriting viruses are dangerous because of their highly destructive effect. These are the easiest ones to discover and are the simplest form of viruses.

When an infected program is executed, the virus searches for new infect able files and either overwrites the whole file or just a part of it (usually the beginning) with a specific malicious code.

The attacked file will be irreparably damaged and will no longer work properly, and as the infected code has been run the host system will be compromised.

Another important type of virus is the Prepender. This type of computer virus adds itself to the beginning of any infectable file just like a destructive virus.

When you run the infected file, the virus is only activated but remains dormant. Thereafter, the virus is in memory of the original state of the host program, and runs from it.

Except for a small slow down, the user does not realize that a virus was active just because the host file is perfectly capable of working.

Appender is another type of harmful virus. An appender virus adds itself to the end a host file and manipulates the infected file so that it will be executed before the program is run.

After the virus has become active, it executes the original program by jumping to the original program entry point. This form of virus is easier to write than a Prepender because the host program needs minimal change.

Since appenders are easy to implement, they are relatively common. But both two types of viruses are very common.

There are also Companion viruses, these do not infect the executable files themselves, but rename the original file and create a file with the original name that contains only the virus, or, they create a file with a similar name that is executed before the original file.

So it is not a virus in a strict sense, since a host program is manipulated. Under Windows, for example, the Companion viruses turn into an executable EXE file. It is a hidden file, and is created with the same name with the extension “.com”, which will only contain the virus.

In the command line of MS-DOS program, the name is entered without an extension. The operating system first looks for programs with the extension “.Com” and then only looks for after programs with the extension “.Exe”.