Just like the biological viruses found in humans and in animals, computer viruses replicate itself before spreading. A \u200fvirus that is biological in nature replicates itself when inside a human host. Once this virus is mixed into the bloodstream, it is now capable of infecting the entire system of the human host. Then, once it is released, whether through the saliva, blood, or sweat, it then contaminates another person. It does not necessarily mean that if it does not spread, the person becomes well, so an outbreak is possible with the growing number of people infected.
Computer viruses act just like this way. A certain computer virus must first be attached to a host system before it replicates. Once it has infiltrated the computer system, it is now able to replicate into something similar or different from its original form. After infecting the whole computer, this virus is now ready to ramble and spread itself to other computer systems. This virus can now spread in different ways. It could be through the Internet, where information can be shared simultaneously; through the local network, where the whole system of computers has access to different files that are logged into the network; or through external methods like a floppy disk or CD.
In order for a virus to replicate, this virus should be given an execute code then committed to memory. Because of this, viruses often fix themselves to files that are part of a legitimate program. When one attempts to run a certain infected program, then the code of this virus will be executed.
Viruses have two sub-divisions, and this is based on their behavior after executed. First is the Resident Viruses and the Nonresident Viruses. Resident Viruses has a replication module just like the nonresident viruses. But the virus' replication module is loaded into the computer memory when one virus is executed to ensure that the virus is able to infect every similar and suitable program to be executed in the computer system.
Resident Viruses have two more category divisions: the Fast Infectors type and the Slow Infectors. Fast Infectors are thus designed so as to infect many files. A fast-infecting virus should infect all accessed files of a potential host. This shows a different problem to the software's anti-virus system program, because the computer system's virus scanner is able to access different potential host files found on one computer whenever it does a scan throughout the system. In the event that the said virus scanner is unable to see if this virus is within the memory, the said virus is able to "piggy-back" or jump onto the scanner to infect all scanned files. Fast infector viruses rely more on their ability to infect at such a fast rate. A disadvantage in this is that viruses would slow down the computer.
On a different note, Slow infecting viruses are made this way to infect the hosts infrequently. For example, slow infecting viruses infect files only when if are copied. Slow infecting viruses are built to avoid any detection through limiting the actions. Because they are unlikely to slow the computer noticeably, it will infrequently trigger the system's software for the anti-virus to detect suspicious actions of the programs. However, in terms of success in infecting files, fast-infecting viruses are more successful and can infect more systems.
Nonresident viruses are viruses that are thought of to have both the replication module and the finder module. For every new file encountered, the replication module is called upon to infect the file. It is the finder module that finds new files that will be infected.
A task of this replicator includes opening a new file, checking if they are already infected, appending a virus code into that file, changing the starting point of this file and points it into the new location of a copied code of that virus, saving the old location to point to a new copied code, saving these changes into the file, closing the said infected file, and returning the file to the finder so it can infect other programs.
Viruses use certain types of system hosts to replicate and execute itself into the computer system. Among them are the Binary Executable Files like the EXE-files or the COM-files in the MS-DOS system, Microsoft Windows, or Linux ELF Files. Others are the floppy disk boot records and hard disk partitions, hard disk master boot record, batch files in Microsoft Windows and MS-DOS, script files of Unix platforms, and VBScript files. Of course, these hosts also include Telix-scripts that are part of Application-specific files, and documents with macros like Microsoft Excel, Word, Access, and AmiPro documents. These hosts are the means of how viruses today replicate.
On one hand, there are viruses called as companion viruses, these old viruses have their own way of replicating that is different with the previously mentioned ones. Companion viruses use MS-DOS and make programs an extension. The other extensions used are .COM and .EXD that are similar with the .EXE file extension. These type of viruses replicate by copying or imitating an authentic program which can be read by MS-DOS. Because it doesn't say which type of file name extension, DOS will automatically run the said file, therefore triggering a virus to run. This will cause this virus to spread into the system before running the real file. These kinds of viruses run under the Windows 95 and DOS emulators programs in the Windows NT system.
It really does not matter how far or slow a virus replicate itself. Once it infiltrates the computer, it will only be a matter of seconds before it does something awful and nasty to the computer system and the stored programs.
Overall, there are two different kinds of viruses depending on its behavior execution. These are the Resident Viruses and the Nonresident Viruses. Both kinds have the replication module and the finder module in infecting a system. Resident Viruses, however, can be subdivided into two categories: the Fast Infectors and the Slow Infectors.
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