How Does Anti-Virus Program Work?

It is noteworthy that each software or program carries an identity sign that is called signature in computer language. All the anti-virus companies keep the dangerous programs in black list and clean program in white list. The companies work with the help of these lists.
When a new program enters in the computer the anti-virus software installed in the computer checks the signatures and looks at the black and white lists. According to an estimate, the black list consists of 94 million dangerous programs while, white list includes 300 million clean programs.
If new program is dangerous according to the list saved in the anti-virus software it is eliminated immediately. If the signatures of new program is not included in the list, anti-virus software can not take care of that and usually asks the user what to do? However, if computer is the part of any network of anti-virus company like Kasper ski or Norton then anti-virus software sends the copy of new program to the company's servers.
The experts of anti-virus company search for the signatures of new suspicious program in their latest lists. If that is not available in their lists then they look at the nature of the program. Whether the program makes illegal changes in the computer and contacts some remote servers or not? After search and investigation if program results in dangerous software then anti-virus software is prepared and is sent to all the costumers. Therefore, it is necessary to update every anti virus program regularly, otherwise it can not identify the fatal program.
The "fertile" brains of American Israelis have prepared very dangerous programs like Stuxnet, flame and Douko that have the unique capability of having the legal signatures that are included in the white list. Therefore the latest anti-softwares could not detect them and they destroyed the targeted computers.