Viral signatures are the patterns used to identify a virus within a file. In order for a virus to be detected, it must be identified and its signature established and distributed. Frequent updates of the data files associated with antiviral software can protect users from known viruses. Unfortunately, there's a significant difference between the time a virus is first released and the time a pattern allowing its detection is available. What is needed, but is extremely difficult to develop, is a way to recognize the character of a virus and differentiate it from normal system activity.
Many viruses attempt to hide themselves.
Others simply store themselves in obscure files and locations. A virus can be triggered in a number of ways. Clearly, no CPU or operating system is going to grab an infected file and execute it. Even a time bomb needs some action to make it run. Because viruses often read people's address books, they will appear to be from known individuals.