It is a (software) program that is installed on a computer without the user's knowledge for the purpose of making harmful changes. A computer virus attaches itself to another program (the execution of the host program triggers the action of the virus). And most of them perform actions that are malicious in nature, such as destroying data. A computer virus operates in two ways: As soon as it lands on a new computer, begins to replicate or it plays dead until the trigger kick starts the malicious code. The term 'computer virus' was formally defined by Fred Cohen in 1983.