A virus is a program which reproduces its own code by attaching itself to other programs in such a way that the virus code is executed when the infected program is executed. Every day we are getting new computer viruses.
A virus reproduces, usually without your permission or knowledge. There are a large number of virus types. Make sure you check your system with the latest products in the market. Be aware of free offers from unknown sites. Always check downloaded files with a powerful AntiVirus program.
Do not forget to check out the sites mentioned in Virus Prevention. Learn how to prevent getting infected and to secure your system as tight as possible. As a reminder: Viruses do not do any harm, ignorance does.
Some computer viruses are programmed to harm your computer by damaging programs, deleting files, or reformatting the hard drive. Others simply replicate themselves or flood a network with traffic, making it impossible to perform any internet activity.
A computer virus is a malware program that is when executed, replicates by inserting copies of itself (possibly modified) into other computer programs, data files, or the boot sector of the hard drive; when this replication succeeds, the affected areas are then said to be "infected".
Different types of Viruses:
Boot sector virus:
This type of virus affects the boot sector of a hard disk. This is done by overwriting or moving the original boot code. After that, they try to move the original code to another sector on the disk and mark that sector as bad.
Examples: Polyboot.B, AntiEXE
A worm is a program very similar to a virus; it has the ability to self-replicate and can lead to negative effects on your system. But they can be detected and eliminated by an antivirus software.
Examples: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson
File infecting virus:
File infecting viruses are the most common ones. They tend to attach or modify all kind of executable files (f.i. 'EXE', '.COM'). A File Deleting Virus is designed to delete critical files which are the part of Operating System or data files.
Macro viruses are written by using the Macro programming languages like VBA, which is a feature of MS office package. A macro is a way to automate and simplify a task that you perform repeatedly in MS office suite (MS Excel, MS word etc).
Examples: Relax, Melissa.A, Bablas, O97M/Y2K
Master Boot Record Infectors:
Infects the system's Master Boot Record on hard drives and the Boot Sector on floppy diskettes. This type of virus takes control of the system at a low level; activating between the system hardware and the operating system. A Boot Sector virus is loaded into memory upon boot-up before virus detection code can be executed.
Polymorphic viruses: Polymorphic viruses encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.
Change their appearance with each infectious action. Can be hard to detect 'cause they are using an encryption algorithm that changes every time. With or after each infection.
Examples: Elkern, Marburg, Satan Bug and Tuareg
Stealth viruses have the capability to hide from the operating system or anti-virus software by making changes to file sizes or directory structure. Stealth viruses are anti-heuristic nature which helps them to hide from heuristic detection. These ones try to hide from the operating system and anti-virus products. Doing this by residing in the memory and intercepting all operating system access.
Another unsavory breed of malicious code are Trojans or Trojan horses, which unlike viruses, do not reproduce by infecting other files, nor do they self-replicate like worms.
A Trojan horse, or Trojan, is a non-self-replicating type of malware which appears to perform a desirable function but instead facilitates unauthorized access to the user's computer system. Trojans do not attempt to inject themselves into other files like a computer virus. Trojan horses may steal information, or harm their host computer systems.
Most common for this type is a combination of techniques of both boot sector viruses and file infecting viruses. Appearance in any combination of the viruses listed above is possible though. F.i. the famous Hare virus (1996) is a polymorphic, stealth, multi-partite virus.
Companion viruses can be considered as a type of file infector virus, like a resident or direct action types. They are known as companion viruses because once they get into the system they 'accompany' the other files that already exist.
Examples: Stator, Asimov.1539 and Terrax.1069
How can we protect our computer from viruses?
Install an antivirus program and keep it updated.
Keeping antivirus programs updated is imperative. Because new viruses are released every day, there's always some risk that your computer will be infected by a virus that your antivirus program does not "know" about.
Unless a rapidly-spreading virus is released, you should be reasonably safe if you update your antivirus program weekly.