A virus is a program that is written which can copy itself and has the capability to completely destroy a computer.A true virus can spread from one computer to another if it is targeting the host computer by the infecting the files in the host computer which is on a network.
Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel and spread throughout Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most could also spread to Macintosh computers. Although most of these viruses did not have the ability to send infected e-mail, those viruses which did take advantage of the Microsoft Outlook.
A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating. Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses
Strategies of Infecting or executing:
Virus need to have an executable file for which they attach to most of the executable files. Viruses can be divided into two types based on their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected and infects those targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.
Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors. Fast infectors are designed to infect as many files as possible. A fast infector, for instance, can infect every potential host file that is accessed. This poses a special problem when using anti-virus software, since a virus scanner will access every potential host file on a computer when it performs a system-wide scan and they infect on the strategy of fast detection rate which is why are also being detected easily because of heuristic behaviour. Slow infectors rely on the strategy of infecting host files
- Binary executable files
- Master Boot record (MBR)
- Script files that include batch files , application specific files, autorun files that will be generated in USB storage devices.
- Cross site Scripting that affects when we visit different websites.
- It also has a capability of infecting to any pdf file by linking a malicious code to it.