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A resident virus is a kind of computer virus that hides and stores itself within the computer memory, which then allows it to infect any file that is run by the computer, depending on the virus’ programming. A resident virus will load its replication module into memory so it does not need to be executed for it to infect other files, rather it activates whenever the operating system loads or operates a specific function. This virus may be one of the worst kinds as they can affect the system thoroughly even attaching itself to anti-virus applications which allows it to infect any file scanned by the program.
Resident viruses are in contrast to non-resident viruses, which are executable viruses meaning that they need to be executed before they can infect, unlike the resident virus that can execute whenever the OS loads. A resident virus has two types: fast infectors and slow infectors, which are both rather self-explanatory. Fast infectors do massive damage quickly, but are very easy to notice because of the effects, while slow infectors are able to spread more widely because they can go undetected for much longer.
Removal of such viruses can be a bit tricky since it has already embedded itself into the computer’s memory. It may even be designed to block the actions of antivirus programs. A special virus removal tool that can extract the virus from the memory is required in most cases. It may be in the form of an OS patch or updates to existing virus removal software. In worst cases, an expert needs to be called to remove the virus without performing a system wipe or reformatting of disks.