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Computer viruses are small type software programs that are designed to spread from one computer to another for corrupt or delete data on your computer, it use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.
Types of Viruses:
1. Resident Viruses
Resident viruses are the other basic level of file infectors. These type of viruses not infect your computer direct, infact they install themselves on computer. It allows them to work even when the original source of the infection has been eradicated. On the basic of the programming of the virus, they can be tricky to spot and even trickier to remove. You can split resident viruses into two areas; fast infectors and slow infectors. Slow infectors are harder to recognize because their symptoms develop slowly, fast infectors cause as much damage as quickly as possible and are thus easier to spot.
2. Non-Resident Viruses
Non-Resident 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).This viruses immediately search for other hosts that can be infected, infect those targets, and finally transfer control to the application program they infected. Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder module is responsible for finding new files to infect.
3. Macro Virus
A macro virus is a computer virus that “infects” a Microsoft Word or similar application and causes a sequence of actions to be performed automatically when the application is started or something else triggers it. Macro viruses tend to be surprising but relatively harmless. A typical effect is the undesired insertion of some comic text at certain points when writing a line. This virus is often spread as an e-mail virus.
Examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K.
4. Boot Sector Virus
From the view of user, boot sector viruses are some of the most dangerous. Because they infect the master boot record, they are notoriously difficult to remove, often requiring a full system format. This is true if the virus has encrypted the boot sector or excessively damaged the code. They typically spread via removable media. They reached a peak in the 1990s when floppy disks were the norm, but you can still find them on USB drives and in email attachments. Luckily, improvements in BIOS architecture have reduced their prevalence in the last few years.
5. Encrypted Viruses
This type of viruses include 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.
6. Polymorphic Virus
According to Symantec, polymorphic viruses are one of the most difficult to detect by an anti-virus program. It claims anti-virus firms need to “spend days or months creating the detection routines needed to catch a single polymorphic”. But why are they so hard to protect against? The clue is in the name. Only one variant of a virus can be blacklist by anti-virus software– but a polymorphic virus changes its signature (binary pattern) every time it replicates. To an anti-virus program, it looks like an entirely different piece of software, and can, therefore, elude the blacklist.
7. Metamorphic Virus
Some of the viruses rewrite themselves completely every time they infect. These viruses are said to be metamorphic. As you might expect, the programming necessary to do this is quite complex and a metamorphic engine attached to a virus may make it large and complex. This process is done on the entire virus therefore the engine also undergoes changes and the whole virus changes with it. The change is made, of course, to attempt to avoid scanners. Don’t be confuse by metamorphic and polymorphic viruses. A polymorphic virus encrypts its original code to protect from pattern recognition and the metamorphic virus changes its code to an equal form.
A computer worm is a selfdependent malware computer program that replicates itself in order to spread to other computers. Often, it uses a computer network to spread itself, relying on security failures on the target computer to access it. Worm use the bandwidth to transfer themself, but they almost always at least harm to the network, whereas viruses almost always corrupt or modify files on a targeted computer. Many of worms have been created are designed only to spread, and do not attempt to change the systems they pass through. If these worms effect on those place where they passed then they can infect major disruption by increasing network traffic and other unintended effects.
Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson.
9. Network Viruses
Network viruses use to rapidly 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 a virus infects a computer, then it search a new target to attack by the of network. When a virus infect a computer then it move to it next target to infext and this process is moving on repeatedly.
The most dangerous network viruses are Nimda and SQLSlammer.
10. Trojans or Trojan Horses
A Trojan horse or Trojan is a type of malware that is often disguised as legitimate software. Trojans can be employed by cyber-thieves and hackers trying to gain access to users’ systems. Users are typically tricked by some form of social engineering into loading and executing Trojans on their systems. Once activated, Trojans can enable cyber-criminals to spy on you, steal your sensitive data, and gain backdoor access to your system.