A resident virus is a computer virus which embeds itself into the memory on a computer, activating whenever the operating system performs a specific function so that it can infect files on the computer. This method of viral infection is in contrast with a non-resident virus, which actively seeks out files to infect. Resident viruses can be quite pernicious, as they may spread through a system so thoroughly that they even attach to antivirus programs, infecting the very things they scan for signs of viral infection.
There are a number of ways to design a resident virus. Virus designers must think about the rate of infection when developing a virus. Some are fast infectors, moving as quickly as possible through a system with the goal of doing the most damage. Others move more slowly. Viruses which move quickly tend to reveal themselves because they cause problems in operating system performance, but this may not be an issue in the eye of the creator since the damage is already known. Viruses which move slowly can be better positioned to infect other computers or to remain hidden for an extended period.
Removing a resident virus which has embedded itself in a computer's memory can be a challenge. The virus may be designed to resist the actions of conventional antivirus software, or as discussed above, to exploit the software. A specialized virus removal tool may be needed to extract the virus from memory. In some cases, the services of an information technology professional may be needed to completely clear a computer of infection.
When a resident virus is identified by an antivirus company or a designer of operating systems, a patch is often released. This may be an update to an antivirus program which allows the program to remove the virus, or it may take the form of a virus removal tool which the computer user can run to get the resident virus out of memory.
Seeking out virus fighting tools can be challenging. Unscrupulous people may release programs which claim to fix viruses, but actually load more viruses or other malicious programs such as spyware onto a computer. Computer users should seek out reputable sources of advice and virus removal software such as official websites for operating system manufacturers or antivirus programs. It is wise to get into the habit of checking the browser's address bar to confirm that one is on the right site before starting a download or filling out information.