What is a computer virus? A computer virus is a type of malware that infects a computer system, mobile device, or other electronic device and can replicate itself to other systems. The most common types of viruses are boot sector viruses, file attachment viruses, and malicious code. Boot sector viruses infect the master boot record (MBR) of the hard drive and affect only operating systems that use MBR. File attachment viruses attach themselves to executable files and spread through removable media such as USB drives. Malicious code attacks random files or folders on your computer system and attempts to install a form of ransomware known as ransomware.