This document discusses computer viruses including their history, symptoms, risky file types, and types of viruses. It provides details on some of the earliest computer viruses from the 1980s and 1990s. It describes common symptoms of a virus-infected computer like slow performance, crashes, and missing files. Risky file types that could contain viruses are listed as .exe, .pif, .bat, .vbs, and .com files. The different types of viruses are outlined such as file infector viruses, macro viruses, and polymorphic viruses. Examples are provided for some virus types like boot sector viruses and web scripting viruses.