The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. Resident viruses do not search for hosts when they are started. Nonresident viruses immediately search for other hosts that can be infected, infect those targets, and finally transfer control to the application program they infected. Viruses can be divided into two types based on their behavior when they are executed.

If a user attempts to launch an infected program, the virus’ code may be executed simultaneously. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. In order to replicate itself, a virus must be permitted to execute code and write to memory. Some viruses and other malware have symptoms noticeable to the computer user, but many are surreptitious or go unnoticed. Worms and Trojans, like viruses, may cause harm to either a computer system’s hosted data, functional performance, or networking throughput, when they are executed.

A worm can exploit security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a program that appears harmless but has a hidden agenda. Viruses are sometimes confused with computer worms and Trojan horses, which are technically different. Malware includes computer viruses, worms, trojans, most rootkits, spyware, dishonest adware, crimeware, and other malicious and unwanted software, including true viruses. The term “computer virus” is sometimes used as a catch-all phrase to include all types of malware, adware, and spyware programs that do not have the reproductive ability. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

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