There are many types of viruses, some cause harmless but some cause devastating. They are classified into difference categories including Boot Sector Viruses, Companion Viruses, Encrypted Viruses, Logic Bomb Viruses, Macro Viruses, Multipartite Viruses, Nonresident Viruses, Polymorphic Viruses, Resident Viruses and Stealth Viruses. You can see the explanation of each type of viruses in following:
1. Boot Sector Viruses
This type of viruses has ability to hide in boot sector. The viruses will load into memory when there is booting system and trying to read from hard disk. Boot sector viruses are more spread since old time when floppy disk was popular. But now we hardly seen them since many of them only can spread through floppy disk.
2. Companion Viruses
Companion Viruses is another kind of viruses. When user computer infect by this sort of viruses, it will create another type file from an existing file in same directory (such as creating file.com from file.exe in the same folder), some companion viruses create file.exe from any folder.
3. Encrypted Viruses
This type of viruses consists of encrypted malicious code, decrypted module. The viruses use encrypted code technique which make antivirus software hardly to detect them. The antivirus program usually can detect this type of viruses when they try spread by decrypted themselves.
4. Logic Bomb Viruses
Logic Bomb Viruses or sometime know as Time Bomb is small piece of malicious code or program which have ability to insert itself to other programs or system and perform specific action when the conditions are met (most Logic Bomb developers use date as conditions). The Logic Bomb does nothing until pre-programmed date is reached. Logic Bomb can perform any malicious things based on pre-programmed within it such as deleting file or displaying unwanted message or lock program and so on.
5. Macro Viruses
When talking about Macro Viruses, we refer to viruses which infect macro of other applications such as Microsoft Word, Microsoft Excel. The viruses are written in a macro language and use it to distribute themselves. Macro viruses will run automatically when user open document. Usually this type of virus cause harmless to your computer, but instead they are annoying by automatically inserting undesired texts or symbols.
Example of Macro Virus: WM.Concept, it was introduced in 1995 the first macro virus that spread through Microsoft Word. And another popular one is Melissa that is first found in 1999, it also can spread through MS Word, Excel and Outlook.
6. Multipartite Viruses
Multipartite Viruses is type of viruses which infect user computer on both part boot sector and executable files and programs at the same time, with this condition, the viruses spread faster than boot sector or file infector alone.
Example: Ghostball, the first multipartite virus.
7. Nonresident Viruses
This type of viruses is similar to Resident Viruses by using replication of module. Besides that, Nonresident Viruses role as finder module which can infect to files when it found one (it will select one or more files to infect each time the module is executed).
8. Polymorphic Viruses:
Polymorphic Virus is similar to encrypted viruses; it can infect files with an encrypted copy of itself. The viruses use difference technique to replicate themselves. Some polymorphic viruses are hardly to detect by antivirus software using virus signature based, because it do not remain any identical after replication.
Example: Pseudonym, 1260
9. Resident Viruses
Resident Viruses or known as Memory Resident Viruses is malicious module. The viruses can replicate module and installing malicious code into computer memory (RAM). The viruses are commonly classified into two main categories: Fast Infectors and Slow Infectors.
10. Stealth Viruses
Stealth Viruses is some sort of viruses which try to trick anti-virus software by intercepting its requests to the operating system. It has ability to hide itself from some antivirus software programs. Therefore, some antivirus program cannot detect them.
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