What Is a Computer Virus?
A computer virus is a small program that attaches itself to other pieces of computer code (like a document or application) and runs without your permission or knowledge. It spreads when these “infected” files are shared with another computer.
A virus is also a generic term for any kind of “malware” – bad stuff that can end up on your computer like worms, trojan horses, or spyware.
How Do You Get Infected?
Your computer can become infected if you open a document or application that is infected with the virus. If your Grandma Martha has an infection on her computer, you could become infected if she sends you an infected document and you open it.
What Can a Virus Do to My Computer?
Some viruses are destructive and can delete files from your computer or erase your hard drive. Other viruses do stupid things like showing a message or playing a sound on a certain date.
A famous virus was named Chernobyl because the main variant became active on April 26, 1999, the 13th anniversary of the Chernobyl disaster. When April 26, 1999 arrived, the virus erased critical parts of hard drives and system BIOS, virtually erasing hard drives and making the computer unable to boot up. Kind of like a mini virtual Chernobyl. Ouch!
How Can You Remove a Virus?
We recommend that you keep your operating system updated automatically (instructions for Windows computers). There are many anti-virus products on the market that will scan for viruses, remove them, and protect you from becoming infected in the future. We’ve tested many of the programs out there and we strongly recommend using ZoneAlarm Security Suite.
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Long Description (from Wikipedia)
In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of the virus into a program is termed infection, and the infected file (or executable code that is not part of a file) is called a host. Viruses are one of the several types of malware or malicious software. In common parlance, the term virus is often extended to refer to computer worms and other sorts of malware. This can confuse computer users, since viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware such as worms.
This confusion can have serious consequences, because it may lead to a focus on preventing one genre of malware over another, potentially leaving computers vulnerable to future damage. However, a basic ule is that computer viruses cannot directly damage hardware, but only software.
While viruses can be intentionally destructive (for example, by destroying data), many other viruses are fairly benign or merely annoying. Some viruses have a delayed payload, which is sometimes called a bomb. For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. A time bomb occurs during a particular date or time, and a logic bomb occurs when the user of a computer takes an action that triggers the bomb. However, the predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.
Currently viruses are somewhat less common than network-borne worms, due to the popularity of the Internet. Anti-virus software, originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware.
A virus is a type of program that can replicate itself by making (possibly modified) copies of itself. The main criterion for classifying a piece of executable code as a virus is that it spreads itself by means of ‘hosts’. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable media. Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with worms.
A worm, however, can spread itself to other computers without needing to be transferred as part of a host. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. Today’s viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms.
Viruses can infect different types of hosts. The most common targets are executable files that contain application software or parts of the operating system. Viruses have also infected the executable boot sectors of floppy disks, script files of application programs, and documents that can contain macro scripts. Additionally, viruses can infect files in other ways than simply inserting a copy of their code into the code of the host program. For example, a virus can overwrite its host with the virus code, or it can use a trick to ensure that the virus program is executed when the user wants to execute the (unmodified) host program. Viruses have existed for many different operating systems, including MS-DOS, AmigaOS, MacOS and even Linux; however, the vast majority of viruses affect Microsoft Windows
A legitimate application program that can copy itself as a side-effect of its normal function (e.g. backup software) is not considered a virus. Some programs that were apparently intended as viruses cannot reliably self-replicate, because the infection routine contains bugs. For example, a buggy virus can insert copies of itself into host programs, but these copies never get executed and are thus unable to spread the virus. Self-replicating programs that have very limited spreading capabilities because of bugs should not be considered legitimate viruses.