As the name suggests, viruses replicate themselves and spread throughout your computer’s memory. Some viruses are made to cripple the users system with malicious code that can affect your files and over time may result in things such as your computer booting slowly. Others are simply made to be irritating and disable useful features such as the task manager. Sometimes viruses do nothing to call attention to themselves and simply reproduce.
Viruses require a human action in order to activate them. Most commonly, viruses attach themselves to an executable file (.exe) because these types of files need to be clicked on in order to run. So this means viruses are generally speaking unintentionally initiated by the user. This is why it is important to ensure you always download files from a trusted source or site.