Computer antivirus companies refer to a virus that has been captured and processed as a known virus, otherwise it is an unknown virus  So far, the prevention of new viruses by antivirus software lags behind the emergence of viruses, because antivirus software can not know the situation of new viruses in advance
99% of the new viruses are written in imitation of known viruses, and the characteristics of their transmission, infection, addition, destruction and other behaviors can be learned from known viruses
 It is impossible for someone to develop an antivirus tool to actively defend against a new virus, which is as whimsical as trying to make a specific medicine for an unknown disease