These are types of viruses that write themselves into system/boot files and affect the operation of the computer due to this.
File viruses write themselves directly into the system file and are embedded in the operability of RAM by performing various destructive actions against the user. An example of such an action may be a slow PC performance.
Boot viruses write themselves into the sectors of the hard disk and perform their boot simultaneously with the boot of the PC system, it is this type of virus that can affect whether the phone number list computer system will turn on or not. If this virus is prescribed so that when the system starts, the user's computer immediately turns off, then this is exactly what will happen, and the user will not have time to do anything.
This type of virus affects only certain systems, for example, a virus can infect Linux, but it will be useless in the Windows OS family because completely different processes operate there.
Technologies used
Virus polymorphism - Used to reduce the level of detection by antivirus programs. Polymorphism itself consists in writing a program on-the-go from the moment of launch, and every time the program is started, a new code is written that does not allow the program to be detected.
Stealth viruses - partially or completely hides its presence in the system by intercepting OS functions / read-install and other things. For example, file-stealth viruses intercept the read/install function in order to hide the increase in infected programs.