This document discusses computer viruses and antivirus software. It defines a computer virus as a program that can copy itself and infect other applications or files. It then lists and describes different types of viruses. The document also defines antivirus software as programs used to prevent, detect, and remove malware like viruses and Trojans. It discusses different types of antivirus software including firewalls, network layer firewalls, application layer firewalls, and proxy servers. The document concludes that antivirus software has become essential to protect computers from threats and ensure optimal performance.