A virus is a type of malware that infects a computer system by attaching itself to another program or file. Once executed, the virus can replicate itself and spread to other files or systems. A virus can be used to gain unauthorized access to a computer system by exploiting software vulnerabilities, stealing credentials, or installing backdoors. A virus can also cause damage to the system by deleting, modifying, or encrypting data, or consuming system resources. According to the Implementing and Operating Cisco Security Core Technologies (SCOR) course, viruses are one of the most common forms of malware and can be classified into different types based on their behavior, such as boot sector viruses, file infectors, macro viruses, or polymorphic viruses1. References: 1: Implementing and Operating Cisco Security Core Technologies (SCOR) course, Module 1: Malware Analysis, Lesson 1: Malware Types and Characteristics, Topic: Virus.