What is a computer virus? A computer virus is a computer program that can copy itself and infect your computer. A virus can spread from one computer to another when its host is taken to the target computer, example if a user sent a infected document over the network or Internet or on removable medium like a CD or Flash drive. Viruses increase their chances of spreading to other computers by infection files on the network file system. Virus can be defined as a set of computer instructions that reproduces itself and it may attach to other executable codes. This code is usually short programs that may either be embedded in other codes or stand on its own. This computer program is designed to infect some aspect of the host computer and then copy itself as much and as often as possible. It's estimated that there are tens of thousands of worms and viruses being spread via the internet and new ones being discovered every day. Most are spread in normal internet activities that are widely used everyday like the sharing of music files, photos, and others such files.
What does a virus do? Not all viruses are always destructive like deleting files or causing your hard drive to crash. A lot of viruses today are more interested in harvesting information from your computer and using it for sending spam or other illegal purposes. Past virus were used to do damage just for the sake of destruction or just to be able to brag about it to peers. Today viruses are widespread and have a root in an economic agenda.
How are virus spreads? The intention of computer viruses is to get it installed in a computer so it can spread itself. This can happen through exposure when computer defenses are down or nonexistent. A virus can spread rapidly and are not easily removed. Virus operates stealthy, coupled with rapid communications that happen between computers today; it's easy to increase the speed that a virus can move around the web and the world. A virus that goes after a network computer can be easily spread since computers are connected and have some vulnerability and access to each other. Viruses will often spread through shared folder, such as emails or other media. Because there are so many types of computer viruses, a virus can infect another computer unintentionally at anytime that a program is running and the virus is activated. Opening a e-mail attachment, downloading a file from the internet, or receiving a copy of a program or file off a flash drive can expose your computer to a virus. The most common way that a virus is spread is e-mail attachments or the use of transfer files using instant messaging.
Viruses are classified as nonresident or resident. The nonresident viruses consist of a finder module and replication module. With the finder module finding new executable files to infect and the replication module infects the file. The resident viruses have replication modules that are similar to the nonresident, but not called by the finder module. The virus loads the replication module into memory when it's executed to ensure that the module is executed each time the operating system carries out certain operations.
There are six broad categories or types of viruses.
Boot Sector Viruses: Are viruses that aim at the boot sector of a hard drive and infect a crucial component of the boot process. The boot sector holds the critical information that controls the hard drive and part of the operating program that is in charge of the whole boot process. These types of viruses load into the system memory while the boot cycle is starting. This virus doesn't affect files, it goes after the drive itself where the virus is saved and this is part of the reason that is no longer as big a threat as it used to be. Since the advent of CDs and DVDs and the drives that carry them it's not possible to infect programs that carry them. With floppy drives the virus could spread quickly from computer to computer by the disks but CDs and DVDs can't be infected this virus can't be spread that way. Another reason the boot sector virus is less common is because the operating systems stand guard over the boot sector and make it hard for the virus to have effect. Some examples of a Boot virus are Form, Disk Killer, Michelangelo, and Stone virus.