What Is Computer Virus ?

Computer Virus is a type of malicious software program that when enter in our devce it starts to spread itself across our system through creating copy of itself by modifying the programs of computer and inserting its own code . When its replication process completed then a part of  get affected that part is known as infected part.Hence A computer virus can be defined as a type of malicious code or program written to alter the way a computer operates and that is designed to spread from one computer to another. A virus operates by inserting or attaching itself to a legitimate program or document that supports macros in order to execute its code. In the process a virus has the potential to cause unexpected or damaging effects, such as harming the system software by corrupting .

Different Types Of Computer Virus :-

Spacefiller Virus

 These type of viruses are also called “Cavity Viruses”. A typical modus operandi for a virus is to simply attach itself to a file, but spacefillers try to get into the empty space which can sometimes be found within the file itself.This method allows it to infect a program without damaging the code or increasing its size, thus enabling it to bypass the need for the stealthy anti-detection techniques other viruses rely on.Luckily, this type of virus is relatively rare, though the growth of Windows Portable Executable files is giving them a new lease of life.

 Encrypted Virus

These viruses contain the encrypted malicious code, decrypted module.Hence we can say that these type of  viruses use the encrypted code technique due to which for antivirus it is hard to detect thes viruses. The antivirus program will able to  detect this type of viruses when they try spread by decrypted themselves.

Companion Virus

Companion viruses are file infector viruses similar resident or direct action viruses. They are known as companion viruses because once they get into the system they "accompany" the other files that already exist. In other words, in order to carry out their infection routines, companion viruses can wait in memory until a program is run (resident viruses) or act immediately by making copies of themselves (direct action viruses).Some examples include: Stator, Asimov.1539, and Terrax.1069

Related Articles :-
What Is Computer Virus ? Different Types Of Computer Virus .  
 What Is Trojan Horse ? Types Of Trojan Horse .
What Is Computer Worm ? Different Types Of Computer Worms . 
What Is Spyware ? Types Of Spyware and Effects Of Spyware .
Network Virus

These type of viruses spread through a Local Network Area (LAN), and sometimes throughout the internet. Generally, network viruses multiply through shared resources, i.e., shared drives and folders. When the virus infects a computer, it searches through the network to attack its new potential prey. When the virus finishes infecting that computer, it moves on to the next and the cycle repeats itself.The most dangerous network viruses are Nimda and SQLSlammer.

Macro Virus

 These viruses specifically target macro language commands in applications like Microsoft Word and other programs. In Word, macros are saved sequences for commands or keystrokes that are embedded in the documents. Macro viruses can add their malicious code to the legitimate macro sequences in a Word file. Microsoft disabled macros by default in more recent versions of Word; as a result, hackers have used social engineering schemes to convince targeted users to enable macros and launch the virus. As macro viruses have seen a resurgence in recent years, Microsoft added a new feature in Office 2016 that allows security managers to selectively enable macro use for trusted workflows only, as well as block macros across an organization.

File Infectors

These type of viruses called file infectors they attach themselves to program files, usually selected .com or .exe files. Some can infect any program for which execution is requested, including .sys, .ovl, .prg, and .mnu files. When the program is loaded, the virus is loaded as well. Other file infector viruses arrive as wholly contained programs or scripts sent as an attachment to an email note.

Nonresident Virus

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).

 Stealth Virus

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.

 Sparse Infectors

In order to spread widely, a virus must attempt to avoid detection. To minimize the probability of its being discovered a virus could use any number of different techniques. It might, for example, only infect every 20th time a file is executed; it might only infect files whose lengths are within narrowly defined ranges or whose names begin with letters in a certain range of the alphabet. There are many other possibilities.

Overwrite Virus

Some viruses are designed specifically to destroy a file or application's data. After infecting a system, an overwrite virus begins overwriting files with its own code. These viruses can target specific files or applications or systematically overwrite all files on an infected device. An overwrite virus can install new code in files and applications that programs them to spread the virus to additional files, applications and systems.

System or Boot-Tecord Infectors

 These viruses infect executable code found in certain system areas on a disk. They attach to the DOS bootsector on diskettes and USB thumb drives or the Master Boot Record on hard disks. In a typical attack scenario, the victim receives storage device that contains a boot disk virus. When the victim's operating system is running, files on the external storage device can infect the system; rebooting the system will trigger the boot disk virus. An infected storage device connected to a computer can modify or even replace the existing boot code on the infected system so that when the system is booted next, the virus will be loaded and run immediately as part of the master boot record. Boot viruses are less common now as today's devices rely less on physical storage media.

Multipartive Virus

While some viruses are happy to spread via one method or deliver a single payload, Multipartite viruses want it all. A virus of this type may spread in multiple ways, and it may take different actions on an infected computer depending on variables, such as the operating system installed or the existence of certain files.They can simultaneously infect both the boot sector and executable files, allowing them to act quickly and spread rapidly.The two-pronged attack makes them tough to remove. Even if you clean a machine’s program files, if the virus remains in the boot sector, it will immediately reproduce once you turn on the computer again.

FAT Virus

The file allocation table or FAT is the part of a disk used to connect information and is a vital part of the normal functioning of the computer. This type of virus attack can be especially dangerous, by preventing access to certain sections of the disk where important files are stored. Damage caused can result in information losses from individual files or even entire directories.

Polymorphic Virus

A polymorphic virus is a type of malware that has the ability to change or mutate its underlying code without changing its basic functions or features. This process helps a virus evade detection from many antimalware and threat detection products that rely on identifying signatures of malware; once a polymorphic virus' signature is identified by a security product, the virus can then alter itself so that it will no longer be detected using that signature.

Resident Virus

 This type of virus embeds itself in the memory of a system. The original virus program isn't needed to infect new files or applications; even if the original virus is deleted, the version stored in memory can be activated when the operating system loads a specific application or function. Resident viruses are problematic because they can evade antivirus and antimalware software by hiding in the system's RAM.
Related Articles :- What Is Adware ? Different Types Of Adware .What Is Rootkit ? Different Types Of Rootkits . 
What Is Ransomware ? Different Types Of Ransomware .What Is Scareware ? Effects Of Scareware .

Causes Of Computer Virus:-

Suspicious Email Attachments

It’s pretty self-explanatory. Many people make the mistake of opening an email that looks shady just to see what it’s all about. That’s especially true if the Subject Line is very attention-grabbing.  As a business owner, and as a computer user, you should never do that.If you ever get an email from an unknown source, don’t click on it. Doing so may put your company data at risk. More often than not, computer virus infections are placed in the attachment files. Alternatively, they come in the form of a website/link contained in the email.

Connecting Infected Removable Storage To Computer

computer-virus-inf2In the case of a company, this situation is more common than you’d think. After all, using removable storage devices (like Memory Cards or USB Flash Drives) is very convenient.However, it’s enough for one of them to have a virus that could possibly infect the entire network.  That’s why you should always check a removable storage device for any signs of a virus infection first. (proper anti-virus software is critical for this).  It’s also important to teach your employees about this safety precaution as well.Not all people know that computer virus infections can be transmitted through these devices. Taking such a risk just isn’t worth it.

Visiting Unsecured Internet Websites Or Sources

In this day and age, this is the most common source of computer viruses. It’s very easy to get one if you’re not careful.  All it takes is a simple file download or website click, and you’ve got yourself a virus. That’s why you must make it a priority to teach your employees not to access unsecured websites at work. The best course of action is to filter out the web addresses that would be considered unsafe. (a proper firewall with built-in web filtering is important)The same policies are mandatory for files as well. Only company-approved files should be downloaded. It’s always better to take pre-emptive measures than to be sorry.

 Pirated Software

An illegal copy of software is called pirated software. Virus can spread if user installs pirated software that contains a virus. A variety of pirated software is available in CDs and from the internet. Some companies intentionally add virus in the software. The virus is automatically activated if the user uses the software without purchasing license.