What is a Computer Virus????
While there is no widely-accepted definition of the term computer virus, the following loose definition should suffice: A computer virus is executable code that, when run by someone, infects or attaches itself to other executable code in a computer in an effort to reproduce itself. Some computer viruses are malicious, erasing files or locking up systems; others merely present a problem solely through the act of infecting other code. In either case, though, computer virus infections should not go untreated.
Closely related to computer viruses are Trojan Horses and worms.
A Trojan Horse is a program that performs some undesired yet intended action while, or in addition to, pretending to do something else. One common class of Trojans are fake login programs - collecting accounts and passwords by prompting for this info just like a normal login program does. Another is a disk defragger that erases files rather than reorganizing them. A Trojan Horse differs from a virus in that the former does not attempt to reproduce itself.
A Worm is just a self-propagating virus. The Internet Worm from November '88 is a famous example.
How does a computer virus attack?
Once a virus has successfully attached to a program, file, or document, the virus will lie dormant until circumstances cause the computer or device to execute its code. In order for a virus to infect your computer, you have to run the infected program, which in turn causes the virus code to be executed. This means that a virus can remain dormant on your computer, without showing major sings or symptoms. However, once the virus infects your computer, the virus can infect other computers on the same network. Stealing passwords or data, logging keystrokes, corrupting files, spamming your email contacts, and even taking over your machine are just some of the devastating and irritating things a virus can do.
While some viruses can be playful in intent and effect, others can have profound and damaging effects, such as erasing data or causing permanent damage to your hard disk, and worst yet, some are even designed with financial gains in mind.
File infectors
These viruses attach themselves to regular programs, such as COM or EXE files under DOS. Thus, they are invoked each time the infected program is run.
Cluster infectors
They modify the file system so that they are run prior to other programs. Note that, unlike file infectors, they do not actually attach themselves to programs.
Macro viruses
Word processing documents can serve as sources of transmission for viruses that take advantage of the auto-execution macro capabilities in products such as Microsoft Word. Simply by opening an infected document, the virus, written in a product's macro language, can spread.
System infectors
Computer operating systems typically set aside a portion of each disk for code to boot the computer. Under DOS, this section is called a boot sector on floppies or a master boot record (MBR) for hard disks. System infectors store themselves in this area and hence are invoked whenever the disk is used to boot the system.
A virus must be executed by someone, perhaps unwittingly, in order to spread. Period. Some ways in which this occurs include:
Running an infected program
As programs infected with a file infector are run, the virus spreads. For this reason, you should regularly scan for viruses any programs you retrieve from a BBS, the net, a colleague, etc... There are even instances of commercial, shrink-wrapped software that have been infected with viruses!
In today’s constantly connected world you can contract a computer virus in many ways, some more obvious than others. Viruses can be spread through email and text message attachments, Internet file downloads, social media scam links, and even your mobile devices and smartphones can become infected with mobile viruses through shady App downloads. Viruses can hide disguised as attachments of socially shareable content such as funny images, greeting cards, or audio and video files.
To avoid contact with a virus it’s important to exercise caution when surfing the web, downloading files, and opening links or attachments. As a best practice, never download text or email attachments that you’re not expecting, or files from websites you don’t trust.
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