How does computer virus work

Computer's Operating System, Application Software are written in a programming language, for which some special type of coding is done so that it works according to the program or software algorithm, when Virus enters the system, it multiplies itself and Changes in Program and Applications Along with this, Virus keeps on replacing the malicious code of Virus with the original code of the computer, due to which the original programming of the computer changes and many types of problems start coming in it.